Question 1

Mycobacteria on Lowenstein–Jensen medium — Which species produces eugonic (luxuriant) growth on L–J medium under standard conditions?

Accepted Answer

Introduction / Context: Lowenstein–Jensen (L–J) medium is a classic egg-based medium used for culturing mycobacteria. Understanding which species grow luxuriantly (eugonic) versus poorly (dysgonic) aids in preliminary identification of members of the Mycobacterium tuberculosis complex. Given Data / Assumptions: Eugonic growth = abundant, rapid colony development relative to other complex members. We compare M. tuberculosis to M. bovis under standard incubation. Other conditions (e.g., additives) are not considered unless routinely used. Concept / Approach: Mycobacterium tuberculosis is eugonic on L–J medium, producing rough, buff, non-pigmented colonies. In contrast, M. bovis is generally dysgonic, showing sparse growth and sometimes requiring special supplementation. This growth difference, while not definitive alone, is a recognized laboratory clue during species differentiation within the M. tuberculosis complex. Step-by-Step Solution: Recall standard growth descriptors on L–J medium.Associate eugonic growth with M. tuberculosis; dysgonic with M. bovis.Select "Mycobacterium tuberculosis" as the species with eugonic growth. Verification / Alternative check: Biochemical tests (e.g., niacin accumulation positive in M. tuberculosis) and molecular assays confirm species identity beyond growth characteristics. Why Other Options Are Wrong: M. bovis: typically dysgonic; not luxuriant on L–J medium. Both/None: contradict established growth patterns used in preliminary ID. Common Pitfalls: Overinterpreting colony size alone without accounting for incubation time and inoculum size; always corroborate with further tests. Final Answer: Mycobacterium tuberculosis

Question 2

General features of Mycobacterium — Members of the genus Mycobacterium are best described as which combination of characteristics?

Accepted Answer

Introduction / Context: Recognizing the shared properties of Mycobacterium species is essential for laboratory identification and clinical understanding. Their cell wall structure and physiology impart distinctive staining behavior and influence treatment strategies. Given Data / Assumptions: We consider broad, textbook properties common to many Mycobacterium species. Gram stain behavior of mycobacteria is variable but taxonomically they fall within Gram-positive lineage. Acid-fastness is a hallmark due to mycolic acid–rich cell walls. Concept / Approach: Mycobacteria possess thick, lipid-rich cell walls with mycolic acids, conferring acid-fastness (resistance to decolorization by acid–alcohol after Ziehl–Neelsen staining). They are typically non-motile and belong to the Gram-positive actinobacterial group, even though their Gram staining can be irregular or weak due to the waxy wall. Therefore, the best overall descriptor is "all of these." Step-by-Step Solution: Confirm acid-fastness as the defining laboratory trait.Acknowledge taxonomic placement in Gram-positive actinobacteria, despite staining variability.Note typical non-motility among clinically relevant species. Verification / Alternative check: Auramine–rhodamine fluorochrome staining and Ziehl–Neelsen microscopy consistently detect acid-fast bacilli, reinforcing this characteristic profile. Why Other Options Are Wrong: Single-feature answers omit other hallmark traits; mycobacteria are not defined by one property alone. Common Pitfalls: Misinterpreting weak Gram staining as Gram-negative; acid-fast staining is the appropriate method for these organisms. Final Answer: all of these

Question 3

Etiology of human tuberculosis — Which bacteria can cause tuberculosis in humans, either classically or in specific epidemiologic settings?

Accepted Answer

Introduction / Context: Tuberculosis (TB) in humans is primarily caused by Mycobacterium tuberculosis, but other members of the M. tuberculosis complex are also capable of producing disease in people, especially in particular geographic or zoonotic contexts. Awareness of these agents informs diagnosis, public health surveillance, and control strategies. Given Data / Assumptions: M. tuberculosis complex includes M. tuberculosis, M. bovis, M. africanum, among others. Human infection may arise via inhalation (classical pulmonary TB) or, for M. bovis, via zoonotic transmission (e.g., unpasteurized dairy). Regional prevalence varies (e.g., M. africanum in parts of West Africa). Concept / Approach: While M. tuberculosis is the dominant global cause of human TB, M. bovis can infect humans (historically via dairy exposure) and M. africanum is an established human pathogen in certain regions. Therefore, all three listed organisms are recognized causes of human tuberculosis. Step-by-Step Solution: Identify each listed species as a member of the M. tuberculosis complex.Recall documented human disease for M. tuberculosis, M. bovis, and M. africanum.Conclude that all of these can cause TB in humans. Verification / Alternative check: Genotyping and lineage studies (e.g., spoligotyping, whole-genome sequencing) confirm relatedness and human pathogenicity across these species. Why Other Options Are Wrong: Selecting a single species ignores the known zoonotic and regional contributors to human TB. Common Pitfalls: Equating all TB solely with M. tuberculosis; clinicians should consider M. bovis in relevant exposures and M. africanum in travelers or residents from endemic areas. Final Answer: All of these

Question 4

Medical microbiology – Buruli ulcer etiology: In clinical practice and public health examinations, which Mycobacterium species is recognized as the causative agent of Buruli ulcer (a necrotizing cutaneous infection producing undermined ulcers and my…

Accepted Answer

Introduction / Context: Buruli ulcer is a neglected tropical disease characterized by painless, undermined skin ulcers. Accurate identification of the etiological agent is vital for diagnosis, treatment selection, and epidemiological control. This question tests recognition of the specific Mycobacterium species responsible and distinguishes it from other non-tuberculous mycobacteria that cause cutaneous lesions. Given Data / Assumptions: Disease: Buruli ulcer with necrotic skin lesions and extensive subcutaneous involvement. Pathogen candidates listed are non-tuberculous mycobacteria commonly implicated in skin/soft tissue infections. Answer must pinpoint the well-established causative agent. Concept / Approach: The hallmark of Buruli ulcer is the cytotoxic macrolide mycolactone, produced by Mycobacterium ulcerans. Mycolactone drives analgesia (painless lesions) and extensive tissue necrosis. Other species, such as Mycobacterium marinum, cause “fish tank granuloma,” typically presenting as nodular lymphangitis or papules following aquatic exposure, not the extensive undermined ulcers of Buruli disease. Rapid growers like Mycobacterium fortuitum and Mycobacterium chelonae can cause post-traumatic or post-procedural wound infections but are not the classical Buruli agents. Step-by-Step Solution: Identify the signature toxin (mycolactone) and link it to Mycobacterium ulcerans. Differentiate from M. marinum (aquarium granuloma) and rapid growers (M. fortuitum, M. chelonae) that have different clinical patterns. Select the species unequivocally associated with Buruli ulcer. Confirm therapeutic implications: combination antibiotics and surgical care are tailored to M. ulcerans. Verification / Alternative check: Epidemiological clustering in West Africa, Australia, and parts of Asia consistently isolates Mycobacterium ulcerans from lesions; molecular assays detect mycolactone polyketide synthase genes supporting causation. Why Other Options Are Wrong: Mycobacterium marinum causes aquatic-associated skin disease but not Buruli ulcer; Mycobacterium fortuitum and Mycobacterium chelonae are rapid growers tied to device or wound infections; Mycobacterium bovis primarily causes zoonotic tuberculosis and does not produce mycolactone. Common Pitfalls: Confusing any cutaneous NTM infection with Buruli ulcer; Buruli is specifically linked to M. ulcerans and its unique toxin. Final Answer: Mycobacterium ulcerans.

Question 5

Biochemical identification of mycobacteria – nitrate reduction test: Which Mycobacterium species is classically nitrate-reduction positive and therefore helps differentiate it from closely related species during laboratory identification?

Accepted Answer

Introduction / Context: The nitrate reduction test is a classic biochemical assay used in mycobacterial identification. It is especially useful for distinguishing Mycobacterium tuberculosis from other members of the M. tuberculosis complex and from non-tuberculous mycobacteria. Selecting the correct nitrate-positive species is a key step in algorithmic identification. Given Data / Assumptions: Nitrate reduction evaluates the ability to reduce nitrate to nitrite. Mycobacterium tuberculosis is classically positive; Mycobacterium bovis is negative. Many rapid-growing NTMs vary, but common exam focus is M. tuberculosis positivity. Concept / Approach: In standard diagnostic workflows, a nitrate-positive result strongly supports Mycobacterium tuberculosis when correlated with other phenotypic features (e.g., slow growth, niacin test, cord factor). Mycobacterium bovis typically lacks nitrate reductase activity, aiding differentiation within the complex. Mycobacterium chelonae is usually nitrate negative; Mycobacterium avium complex organisms tend to be negative as well. Although some non-tuberculous species can show variable results, the textbook-correct identification target is M. tuberculosis as the nitrate-positive reference. Step-by-Step Solution: Recall nitrate reduction profiles for key species. Eliminate species known to be negative (M. bovis, M. chelonae, many MAC isolates). Select M. tuberculosis as the canonical nitrate-positive species. Integrate with other tests for full speciation in practice. Verification / Alternative check: Laboratory manuals and exam references consistently list M. tuberculosis as nitrate-positive and M. bovis as nitrate-negative, forming a common viva question pair. Why Other Options Are Wrong: M. bovis is nitrate-negative; M. avium is typically negative; M. chelonae is negative; strain variability in some NTM does not override the canonical teaching point. Common Pitfalls: Assuming “all of these” because some rapid growers may reduce nitrate; the reliable exam choice remains M. tuberculosis. Final Answer: Mycobacterium tuberculosis (positive).

Question 6

Environmental (saprophytic) mycobacteria – example: Which organism listed below is a classic example of a saprophytic mycobacterium frequently used in teaching laboratories and molecular work because of its nonpathogenic nature and rapid growth?

Accepted Answer

Introduction / Context: Non-tuberculous mycobacteria (NTM) include both environmental saprophytes and opportunistic pathogens. Knowing which organisms are saprophytic helps with risk assessment and with choosing safe model species for laboratory instruction and genetic studies. Given Data / Assumptions: Saprophytes thrive on decaying organic matter and environmental substrates. Mycobacterium smegmatis is a rapid grower, nonpathogenic, widely used in research. Other listed species have recognized pathogenic potential in humans. Concept / Approach: Mycobacterium smegmatis is a classical saprophytic species, prized for its safety and fast growth rate compared with slow-growing pathogenic mycobacteria. In contrast, Mycobacterium marinum causes fish tank granuloma; Mycobacterium chelonae and related rapid growers cause post-procedural infections; Mycobacterium xenopi and Mycobacterium kansasii are associated with pulmonary disease. Therefore, the best example of a saprophytic, teaching-lab-friendly mycobacterium is M. smegmatis. Step-by-Step Solution: Identify which species is nonpathogenic and used as a model organism. Exclude species known for human disease (skin or pulmonary). Select Mycobacterium smegmatis. Note that its rapid growth eases culture-based exercises. Verification / Alternative check: Research literature and teaching manuals routinely employ M. smegmatis mc^2155 strains for cloning and expression experiments, confirming its saprophytic status and utility. Why Other Options Are Wrong: M. marinum, M. chelonae, M. xenopi, and M. kansasii can cause human disease and are not the typical saprophytic classroom organism. Common Pitfalls: Equating “rapid-growing” with “nonpathogenic.” Several rapid-growing mycobacteria are opportunists; M. smegmatis is the safer, classic saprophyte. Final Answer: Mycobacterium smegmatis.

Question 7

Colony morphology on Löwenstein–Jensen (LJ) medium: Which mycobacterial species classically produces rough, tough, and buff (nonpigmented, wrinkled) colonies on LJ medium during culture?

Accepted Answer

Introduction / Context: Colony morphology on Löwenstein–Jensen medium provides classic visual clues in tuberculosis laboratories. The phrase “rough, tough, and buff” is a time-honored mnemonic for a particular organism’s appearance in culture. Given Data / Assumptions: LJ medium supports growth of slow-growing mycobacteria. “Rough, tough, buff” describes colony texture (wrinkled, resilient) and color (buff, nonchromogenic). Multiple mycobacteria can be cultured, but not all show this hallmark morphology. Concept / Approach: Mycobacterium tuberculosis forms rough, wrinkled, nonchromogenic colonies on LJ medium, often described as “rough, tough, and buff.” Mycobacterium bovis tends to form smaller, more dysgonic colonies; rapid growers such as Mycobacterium fortuitum and Mycobacterium chelonae produce colonies more quickly and with different textures and pigmentation patterns. Therefore, the textbook association belongs to M. tuberculosis. Step-by-Step Solution: Recall the mnemonic linking rough-tough-buff to M. tuberculosis. Exclude species with distinct colony characteristics or growth rates. Select Mycobacterium tuberculosis as the correct match. Corroborate with cord factor presence and niacin test as complementary clues. Verification / Alternative check: Culture atlases consistently photograph M. tuberculosis colonies exhibiting the classic rough, buff appearance on LJ slants. Why Other Options Are Wrong: M. bovis colonies differ in size and are more fastidious; rapid growers yield faster, different-appearing colonies; MAC organisms are often smooth and nonchromogenic but lack the classic R/T/B description. Common Pitfalls: Assuming all slow growers look the same; subtle but reliable differences exist and support preliminary identification. Final Answer: Mycobacterium tuberculosis.

Question 8

Sensitivity of culture methods – bacillary load threshold: Approximately how many tubercle bacilli per millilitre are required in a specimen for routine culture methods to yield a positive result under laboratory conditions?

Accepted Answer

Introduction / Context: Understanding the detection thresholds of smear versus culture enhances interpretation of negative results. Smear microscopy is less sensitive than culture; culture can detect much lower bacillary counts, impacting diagnostic algorithms and infection control decisions. Given Data / Assumptions: Smear positivity often requires roughly 10^4 organisms per mL. Culture methods (solid or liquid media) detect much smaller numbers. Typical exam figures cite ranges rather than a single value. Concept / Approach: Routine culture on solid media such as LJ or, more sensitively, in liquid systems can detect bacillary loads on the order of tens per mL. A commonly taught threshold is 10–100 organisms per mL for culture positivity, whereas smears require far higher counts, explaining smear-negative/culture-positive cases in clinical practice. Step-by-Step Solution: Contrast smear detection (~10^4 per mL) with culture detection (~10–100 per mL). Identify the option matching the accepted culture threshold. Select “As few as 10–100 per mL.” Note: exact sensitivity varies by medium and decontamination procedures. Verification / Alternative check: Clinical guidelines acknowledge smear-negative but culture-positive pulmonary TB, supporting the lower detection limit of culture. Why Other Options Are Wrong: Values of 1–9 per mL are overly optimistic for routine culture; 125–200 per mL is too high and more aligned with less sensitive methods; 10,000 per mL corresponds to smear microscopy needs. Common Pitfalls: Assuming a negative smear excludes TB; culture and molecular tests are needed for low-burden disease. Final Answer: As few as 10–100 per mL.

Question 9

Drug susceptibility profiles – pyrazinamide (PZA): Which mycobacterial species listed is characteristically susceptible to pyrazinamide under standard testing conditions, aiding in species differentiation within the M. tuberculosis complex?

Accepted Answer

Introduction / Context: Pyrazinamide is a first-line antitubercular drug active in acidic environments, particularly effective against semi-dormant bacilli. Recognizing which species are susceptible versus resistant assists in both therapy and species-level differentiation inside the M. tuberculosis complex. Given Data / Assumptions: Mycobacterium tuberculosis is generally PZA-susceptible unless resistant mutants are present. Mycobacterium bovis is inherently PZA-resistant due to pncA-related differences. Many non-tuberculous mycobacteria, including MAC and rapid growers, are not reliably susceptible. Concept / Approach: PZA susceptibility helps distinguish M. tuberculosis (susceptible) from M. bovis (resistant). PZA is a prodrug converted to pyrazinoic acid by pyrazinamidase; functional differences in this enzyme drive the susceptibility profile. Rapid growers and MAC species typically show intrinsic resistance and are not treated with PZA in standard regimens. Step-by-Step Solution: Recall species-specific PZA profiles. Exclude M. bovis and most NTM as resistant/non-reliably susceptible. Select Mycobacterium tuberculosis (susceptible). Link to clinical regimens where PZA is included in intensive phase. Verification / Alternative check: Guideline-based regimens for drug-susceptible pulmonary TB include PZA, contrasting with treatment of zoonotic TB due to M. bovis where PZA is omitted. Why Other Options Are Wrong: M. bovis is classically resistant; M. fortuitum and MAC organisms are not targeted with PZA. Common Pitfalls: Assuming all “TB bacteria” are PZA-susceptible; species-level distinctions matter. Final Answer: Mycobacterium tuberculosis (susceptible).

Question 10

Differentiating M. tuberculosis from M. bovis – TCH susceptibility: Which organism is classically susceptible to thiophen-2-carboxylic acid hydrazide (TCH), a reagent historically used to distinguish members of the M. tuberculosis complex?

Accepted Answer

Introduction / Context: Before widespread molecular typing, biochemical reagents such as TCH aided in distinguishing species within the M. tuberculosis complex. Recognizing the classic TCH susceptibility pattern remains a common viva question in microbiology courses. Given Data / Assumptions: M. bovis: TCH susceptible. M. tuberculosis: TCH resistant. Rapid growers like M. chelonae and M. fortuitum are not in the complex and are not defined by TCH patterns for speciation. Concept / Approach: TCH susceptibility is a traditional phenotypic criterion: growth inhibition by TCH indicates M. bovis, whereas resistance suggests M. tuberculosis. While modern labs rely on molecular assays, the biochemical distinction remains testable knowledge for examinations and historical understanding of TB lab methods. Step-by-Step Solution: Recall MTBC-specific TCH profiles (bovis susceptible; tuberculosis resistant). Exclude rapid growers and variable NTM. Select M. bovis as the TCH-susceptible organism. Note the complementary use of niacin and nitrate tests in classical workflows. Verification / Alternative check: Legacy identification tables consistently document TCH susceptibility for M. bovis as a differentiator from M. tuberculosis. Why Other Options Are Wrong: M. tuberculosis is typically TCH resistant; rapid growers and other NTM are not identified within MTBC schemes by TCH. Common Pitfalls: Assuming all MTBC members behave identically biochemically; subtle differences underpin older speciation methods. Final Answer: Mycobacterium bovis (susceptible).

Question 11

Atmospheric requirements on primary isolation: Which member of the Mycobacterium tuberculosis complex commonly shows microaerophilic tendencies on primary isolation, preferring reduced oxygen tension compared with strict aerobes?

Accepted Answer

Introduction / Context: Mycobacteria are generally considered aerobic; however, subtle differences in oxygen preferences can aid in species recognition, especially during primary isolation from clinical or animal specimens. Given Data / Assumptions: M. tuberculosis is a classic obligate aerobe and grows well under normal atmospheric oxygen. M. bovis is more fastidious and may prefer microaerophilic conditions during early isolation. Rapid-growing mycobacteria like M. fortuitum adapt readily under standard aerobic incubation. Concept / Approach: Historically, M. bovis has been described as microaerophilic or more fastidious than M. tuberculosis, particularly in primary culture from bovine samples. This property, alongside colony morphology and biochemical reactions, was used to guide presumptive identification before genomic tests were routine. Recognizing this nuance helps answer exam questions on growth characteristics. Step-by-Step Solution: Compare oxygen preferences: M. tuberculosis (aerobe) vs M. bovis (more fastidious, microaerophilic tendencies). Exclude rapid growers that do not require lowered oxygen for primary isolation. Select M. bovis as the species fitting the description. Understand that modern automated systems optimize conditions but the classical teaching remains. Verification / Alternative check: Older laboratory manuals note improved primary isolation of M. bovis under reduced oxygen tension compared with M. tuberculosis. Why Other Options Are Wrong: M. tuberculosis is strictly aerobic; M. fortuitum is a rapid-growing aerobe; “none of these” is incorrect as one option fits. Common Pitfalls: Assuming identical atmospheric preferences across all MTBC members; there are meaningful differences. Final Answer: Mycobacterium bovis.

Question 12

Transmission route of human tuberculosis: By which route do humans most commonly acquire infection with Mycobacterium tuberculosis in community and healthcare settings?

Accepted Answer

Introduction / Context: Understanding the predominant transmission route of tuberculosis is essential for infection prevention and control. It guides public health measures such as ventilation, masking, and early detection strategies in high-risk environments. Given Data / Assumptions: Mycobacterium tuberculosis is shed in respiratory secretions from persons with pulmonary TB. Droplet nuclei remain suspended and can be inhaled by close contacts. Other routes exist but are far less common for M. tuberculosis. Concept / Approach: The overwhelming majority of M. tuberculosis infections arise from inhalation of droplet nuclei generated during coughing, speaking, or aerosol-generating procedures. Gastrointestinal acquisition is more relevant to zoonotic mycobacteria (e.g., M. bovis in unpasteurized dairy, historically). Percutaneous inoculation is rare and typically occupational. Contact through intact skin is not a recognized route. Recognizing inhalation as the key route informs airborne precautions (N95/FFP2, negative pressure rooms). Step-by-Step Solution: Identify the clinical syndrome: pulmonary TB as primary source of transmission. Link airborne droplet nuclei to person-to-person spread. Exclude uncommon routes (ingestion, inoculation, intact skin contact). Select inhalation as the correct route. Verification / Alternative check: Epidemiologic investigations and outbreak analyses consistently implicate shared air in enclosed spaces as the driver of transmission, with risk factors including poor ventilation and prolonged exposure. Why Other Options Are Wrong: Ingestion relates more to M. bovis historically; contact via intact skin is ineffective; percutaneous inoculation is rare and occupational; transplacental spread is possible but very uncommon. Common Pitfalls: Overemphasizing fomite/contact routes; M. tuberculosis is primarily airborne. Final Answer: Inhalation of infectious droplet nuclei/aerosols.

Question 13

Historical laboratory epidemiology – phage types of M. tuberculosis in India: According to classical mycobacteriophage typing schemes used historically, which phage type of Mycobacterium tuberculosis was reported most frequently in India?

Accepted Answer

Introduction / Context: Before modern genotyping methods (e.g., spoligotyping, MIRU-VNTR, WGS), some regions used mycobacteriophage typing to categorize M. tuberculosis isolates. Exam questions may reference these historical data, particularly the most frequently observed types in specific countries like India. Given Data / Assumptions: Phage typing assigned isolates into types based on susceptibility patterns to a set of bacteriophages. Indian surveillance historically reported predominance of a specific type. Current practice has largely replaced phage typing with molecular methods, but legacy knowledge persists in exams. Concept / Approach: While not commonly used today, the classical teaching in many South Asian curricula cites Type I as the most frequent phage type of M. tuberculosis in India. This reflects earlier epidemiologic surveys and is preserved in older question banks. Understanding that this is a historical reference prevents confusion with current molecular lineage designations (e.g., Lineage 3 or 4) used in contemporary literature. Step-by-Step Solution: Recognize the question refers to historical phage typing rather than modern genotyping. Recall the commonly cited predominant type in India (Type I). Exclude lettered options (A, B, C) that do not match the conventional numeric “Type I” designation. Select Type I as the correct choice. Verification / Alternative check: Older microbiology texts and exam compilations list Type I as predominant in India under phage typing schemes, although this method is now largely obsolete. Why Other Options Are Wrong: Types A, B, and C either do not match the standard numeric designation or were not reported as predominant; Type III was not the commonly cited majority. Common Pitfalls: Confusing phage typing categories with modern genetic lineages; they are unrelated systems. Final Answer: Type I.

Question 14

Tuberculin skin testing (Mantoux): In standard practice for adults with no special risk factors, which induration size is interpreted as a positive Mantoux test?

Accepted Answer

Introduction / Context: The Mantoux test (tuberculin skin test) measures delayed-type hypersensitivity to purified protein derivative injected intradermally. Reading is based on the transverse diameter of induration, not erythema. Thresholds for a positive result depend on risk category, but a commonly taught cut-off for people without special risk factors is 10 mm or more of induration. Given Data / Assumptions: Population considered: general adults without known high-risk conditions. Measurement is at 48–72 hours after intradermal PPD. Interpretation is by size of induration, not redness. Concept / Approach: Cell-mediated immunity from prior Mycobacterium tuberculosis exposure or BCG vaccination can produce induration. Interpretation uses tiered cut-offs: 5 mm for very high-risk groups (e.g., HIV infection, recent contacts, immunosuppressed), 10 mm for intermediate risk (e.g., recent immigrants, healthcare workers, certain medical conditions), and 15 mm for persons with no known risk. In routine exam questions, 10 mm is the standard positive threshold unless a specific high-risk scenario is stated. Step-by-Step Solution: Identify the group as general risk (no special modifiers given). Recall standard interpretation thresholds for induration. Match the general-risk threshold to 10 mm or more. Select the option that states 10 mm or more in diameter. Verification / Alternative check: Clinical guidelines demonstrate tiered cut-offs. Absent explicit high-risk factors in the stem, examination convention defaults to 10 mm as positive for most adults. Why Other Options Are Wrong: 1–4 mm or 4–9 mm: Typically negative in general-risk individuals. 5 mm for everyone: Applies only to high-risk categories. None of these: Incorrect because 10 mm is a valid positive threshold for many adults. Common Pitfalls: Measuring erythema instead of induration; ignoring patient risk category; reading before 48 hours or after 72 hours reduces accuracy. Final Answer: 10 mm or more in diameter.

Question 15

Mantoux (tuberculin) testing: In which situations are false-negative Mantoux results most likely to occur?

Accepted Answer

Introduction / Context: The Mantoux test depends on intact cell-mediated immunity. Conditions that blunt T-cell responsiveness can lead to false-negative results even when Mycobacterium tuberculosis infection is present. Recognizing these scenarios is crucial for clinical interpretation and public health decisions. Given Data / Assumptions: The question asks about causes of false negativity, not true negativity. Examples include early infection, advanced disease, and malnutrition. Other immunosuppressive states act similarly (e.g., HIV, corticosteroids). Concept / Approach: False negatives arise when delayed-type hypersensitivity is not yet developed (window period) or is impaired by anergy. Early TB (prior to sensitization) may not elicit an induration. In advanced, disseminated TB, immune exhaustion or anergy can reduce responsiveness. Severe protein-calorie malnutrition impairs T-cell function, decreasing the reaction. Therefore, a negative test does not exclude TB in these settings. Step-by-Step Solution: List conditions that suppress DTH responses: early infection, anergy, malnutrition. Relate each to reduced induration despite infection. Recognize all choices describe recognized false-negative scenarios. Choose “All of these.” Verification / Alternative check: Clinical practice uses interferon-gamma release assays or repeats testing after the window period if suspicion remains high; this supports the understanding that false negatives occur in these states. Why Other Options Are Wrong: Each single option is correct but incomplete alone. None of these: Incorrect because all listed conditions can cause false negatives. Common Pitfalls: Over-relying on a single negative Mantoux in high-risk or symptomatic patients; ignoring malnutrition or immune suppression in interpretation. Final Answer: All of these.

Mycobacterium Questions