Question 1

Prontosil (the first sulfonamide): in which setting is it an effective antibacterial agent, and why was this discovery pivotal?

Accepted Answer

Introduction: Prontosil, a red azo dye discovered in the 1930s, ushered in the antibiotic era. Its paradox—active in vivo but inactive in vitro—transformed chemotherapeutic research. This question tests whether you know where Prontosil works and the metabolic basis for its activity. Given Data / Assumptions: Prontosil shows antibacterial efficacy in animals and humans. It does not inhibit bacterial growth in standard cell-free culture tests. Metabolic activation in the host is required for antibacterial effect. Concept / Approach: Prontosil is a prodrug. In vivo, it is metabolized (via azo-reduction) to release sulfanilamide, the true active antibacterial that inhibits folate synthesis (dihydropteroate synthase). Since cell-free culture tests lack this metabolic conversion, Prontosil appears inactive in vitro, explaining the early experimental discrepancy. Step-by-Step Solution: Recall Prontosil’s historical observation: cures infections in animals but fails in test tubes. Identify prodrug nature: metabolic cleavage yields sulfanilamide. Connect mechanism: folate pathway inhibition blocks bacterial growth in vivo. Choose the option indicating in vivo (animals) efficacy only. Verification / Alternative check: Subsequent use of sulfanilamide itself in vitro demonstrates robust antibacterial activity, confirming Prontosil’s prodrug status. Why Other Options Are Wrong: In vitro activity: Absent without metabolic activation. Both in vivo and in vitro: Contradicted by early experimental data. Not used as antibacterial: Historically pivotal as first widely used systemic antibacterial. Common Pitfalls: Assuming all antibiotics show direct in vitro activity; some agents require bioactivation within the host. Final Answer: an effective antibacterial when used in animals — Prontosil is active in vivo due to metabolic conversion to sulfanilamide.

Question 2

Cytokine nomenclature: which cytokine is also known as cachectin because of its role in inducing cachexia and systemic inflammatory responses?

Accepted Answer

Introduction: Cytokines often have multiple historical names reflecting their discovery contexts and bioactivities. This question checks whether you recognize the alias “cachectin” for a key proinflammatory mediator implicated in sepsis, chronic inflammation, and cancer-associated cachexia. Given Data / Assumptions: Cachexia is a metabolic wasting syndrome seen in chronic disease. Proinflammatory cytokines can drive anorexia, muscle proteolysis, and lipolysis. Terminology overlaps occur: one molecule, multiple names. Concept / Approach: Tumor necrosis factor (TNF, especially TNF-alpha) was historically termed cachectin due to its ability to induce cachexia in experimental models. It plays central roles in fever, acute phase responses, and septic shock. Interferon-gamma and interleukin-2 have immunoregulatory roles but are not known as cachectin. Step-by-Step Solution: Recall alternate names: cachectin corresponds to TNF. Differentiate cytokine functions: TNF drives systemic inflammation and cachexia. Exclude IFN-gamma and IL-2 based on nomenclature and biology. Select TNF as the correct answer. Verification / Alternative check: Pharmacologic blockade of TNF (e.g., anti-TNF biologics) reduces inflammatory disease activity, underscoring its systemic effects. Why Other Options Are Wrong: Interferon gamma: Activates macrophages and Th1 responses; not called cachectin. Interleukin 2: T cell growth factor; not referred to as cachectin. None of the above: Incorrect because TNF matches the alias. Common Pitfalls: Confusing cachexia mechanisms; while multiple cytokines contribute, the historical term cachectin specifically denotes TNF. Final Answer: Tumor necrosis factor (TNF) is also known as cachectin.

Question 3

Why do plastic implants (e.g., catheters, prosthetic joints, heart valves) pose a serious nosocomial infection problem in hospitals and clinics?

Accepted Answer

Introduction: Medical devices revolutionize care but create new infection risks. This question integrates biomaterials science, innate immunity, and clinical management to explain why plastic implants are frequent sources of healthcare-associated infections. Given Data / Assumptions: Many pathogens form biofilms on abiotic surfaces (e.g., Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa). Biofilms protect bacteria from host defenses and antibiotics. Mechanical and surgical considerations influence treatment strategies. Concept / Approach: Plastic surfaces permit rapid conditioning by host proteins, followed by bacterial adhesion and biofilm formation. Within biofilms, extracellular polymeric substances impede neutrophil migration and phagocytosis, and diffusion barriers reduce antibiotic penetration. Furthermore, the lack of integrin ligands and the smooth, hydrophobic nature of plastics impair phagocyte motility. Persistent device infections often require device removal because antimicrobial therapy alone rarely eradicates mature biofilms. Step-by-Step Solution: Recognize that biofilm formation is common on plastic devices. Explain that neutrophils/macrophages have difficulty crawling and engaging targets on plastic and within EPS matrices. Link biofilm tolerance to clinical failure of antibiotics. Conclude that surgical explantation is frequently necessary to cure infection. Verification / Alternative check: Clinical guidelines recommend removal of infected central lines or prostheses when feasible, reflecting the recalcitrance of biofilm-associated infections. Why Other Options Are Wrong: Each individual statement is accurate but partial; together they form the complete explanation, making the comprehensive option correct. Common Pitfalls: Overreliance on antibiotics without addressing the device as the nidus leads to recurrent or persistent infections. Final Answer: all of the above — impaired phagocyte function on plastics, biofilm protection, and the need for device removal explain the risk.

Question 4

Innate immunity — Lactoferrin, a multifunctional host glycoprotein found in tears, saliva, milk, and neutrophil granules, helps protect against pathogenic microbes primarily by which mechanism?

Accepted Answer

Introduction / Context: Lactoferrin is an iron-binding glycoprotein that contributes to the innate immune defense on mucosal surfaces and within neutrophil secondary granules. Because many pathogens require freely available iron for growth and for key redox enzymes, limiting iron availability is a powerful, host-driven antimicrobial strategy. This question tests understanding of the primary protective mechanism of lactoferrin and distinguishes it from other nonspecific defenses like acidity or commensal competition. Given Data / Assumptions: Lactoferrin is present in secretions such as tears, saliva, and milk, and is released by activated neutrophils. Pathogens depend on bioavailable ferric or ferrous iron for replication. We compare mechanisms: iron sequestration versus pH change, sebum effects, or commensal growth promotion. Concept / Approach: Lactoferrin tightly chelates iron, reducing the concentration of free iron in the local environment. This creates a bacteriostatic effect because iron-dependent enzymes and respiratory pathways are impaired. In addition, lactoferrin can disrupt biofilms, bind to lipopolysaccharide, and modulate immune responses, but the best known and most fundamental antimicrobial action is nutrient sequestration of iron. Lowering pH is primarily achieved by organic acids and gastric acid, while modulation of sebum is unrelated, and commensal growth promotion is an indirect consequence rather than the main mode of action. Step-by-Step Solution: Identify lactoferrin as an iron-binding glycoprotein that limits iron availability to microbes.Recall that many bacteria upregulate siderophores in response to iron starvation, underscoring the importance of iron in growth.Conclude that iron sequestration is the principal antimicrobial mechanism attributed to lactoferrin. Verification / Alternative check: Experimental systems show growth inhibition of common pathogens in the presence of apolactoferrin; addition of excess iron reverses this inhibition, confirming that iron withdrawal is the key factor rather than direct cytotoxicity. Why Other Options Are Wrong: Blocking sebum production: sebum is a skin lipid secretion; lactoferrin does not block its production. Lowering the pH: acidification is a general defense but is not the principal action of lactoferrin. Facilitating normal flora: commensals may benefit secondarily, but this is not the primary biochemical mechanism. Common Pitfalls: Assuming all secreted antimicrobial proteins act by membrane disruption; lactoferrin mainly starves microbes of iron, with additional modulatory effects. Final Answer: sequestering Iron

Question 5

Fever is not always infectious — Which of the following is a non-infectious cause of fever in clinical practice?

Accepted Answer

Introduction / Context: Fever is a regulated elevation of core body temperature driven by pyrogenic cytokines. While infections are the most common triggers, numerous non-infectious inflammatory and autoimmune conditions also cause fever. Distinguishing infectious from non-infectious etiologies guides diagnostic testing, antimicrobial stewardship, and management decisions. Given Data / Assumptions: Options include classical infectious diseases and one inflammatory autoimmune category. Non-infectious causes can produce systemic inflammation and pyrexia through cytokine release. The question asks specifically for a non-infectious example. Concept / Approach: Typhoid, chicken pox, and malaria are established infectious causes of fever due to bacterial, viral, and parasitic pathogens respectively. In contrast, rheumatic diseases (for example, rheumatoid arthritis, systemic lupus erythematosus, vasculitides) can cause fever through sterile inflammation. Recognition depends on clinical context, exposure history, and laboratory markers (autoantibodies, inflammatory markers) rather than cultures or pathogen detection alone. Step-by-Step Solution: Identify which options are infectious: typhoid (bacterial), chicken pox (viral), malaria (protozoal).Recognize rheumatic disease as a non-infectious, immune-mediated category.Select rheumatic disease as the correct non-infectious cause of fever. Verification / Alternative check: Clinical algorithms for fever of unknown origin always include autoimmune and neoplastic conditions alongside infections and miscellaneous causes, highlighting non-infectious sources. Why Other Options Are Wrong: Typhoid: caused by Salmonella enterica serovar Typhi; infectious. Chicken pox: caused by varicella-zoster virus; infectious. Malaria: caused by Plasmodium species; infectious. Common Pitfalls: Assuming antibiotic therapy is needed for all fevers; in non-infectious etiologies, immunomodulatory therapy may be appropriate after excluding infection. Final Answer: Rheumatic disease

Question 6

Anthrax facts — Select the true statement about Bacillus anthracis and the forms of anthrax in humans.

Accepted Answer

Introduction / Context: Anthrax is a zoonotic disease of herbivores that occasionally affects humans through contact with infected animals or contaminated animal products. The etiologic agent is Bacillus anthracis, a spore-forming Gram-positive bacillus. Clinical forms include cutaneous, inhalational, gastrointestinal, and injection anthrax. Understanding transmission and infectious dose is critical for risk assessment and public health responses. Given Data / Assumptions: We compare correctness of statements about causative agent, contagion, strain differences, and infectious dose. Human-to-human spread is rare to negligible. Spores are the infectious form; dose thresholds matter. Concept / Approach: Bacillus anthracis causes anthrax; it is not viral. Anthrax is typically not spread person to person; most cases result from environmental or occupational exposure to spores. The same species and overlapping lineages cause the various clinical forms depending on route of exposure and dose, rather than distinct specialized strains for each form. Inhalational anthrax requires exposure to a relatively large number of spores to overcome host defenses in the respiratory tract, making the statement about a large infective dose the best true choice here. Step-by-Step Solution: Rule out the viral etiology; anthrax is bacterial.Note low person-to-person transmissibility; contagion is not high.Recognize that clinical form depends on portal of entry, not unique strains.Accept that inhalational disease requires a comparatively large spore dose to establish infection. Verification / Alternative check: Public health guidance describes inhalational anthrax onset after substantial aerosol exposure to spores, with estimates of infective dose being much higher than for many respiratory pathogens. Why Other Options Are Wrong: Virus: incorrect; B. anthracis is a bacterium. Highly contagious: generally false; typical cases do not spread between people. Separate strains for forms: misleading; route and dose drive presentation. Common Pitfalls: Equating environmental stability of spores with ease of person-to-person transmission; the two are distinct. Final Answer: Inhalation Anthrax requires infection with a large number of spores

Question 7

Gastrointestinal defenses — Which of the following is NOT a major innate defense mechanism operating in the stomach?

Accepted Answer

Introduction / Context: The stomach provides one of the body’s most effective microbial barriers. Understanding which factors constitute true defenses clarifies how pathogens must adapt to survive passage into the intestine and why reduced acidity increases infection risk. This question differentiates genuine defenses from features that are absent or minimal in the stomach environment. Given Data / Assumptions: Focus on the stomach, not the colon or oral cavity. Consider chemical and enzymatic barriers and the resident microbiota. Pick the factor that is not a major defense in this specific site. Concept / Approach: Low pH (due to hydrochloric acid) is a principal defense, denaturing proteins and inactivating many microbes. Proteolytic enzymes like pepsin degrade proteins and contribute to microbial killing. In contrast, the stomach does not harbor a dense normal flora because its acidity limits persistent colonization. A dense microbiota is characteristic of the colon, not the stomach. Therefore, “dense normal flora” is not a stomach defense mechanism and is the correct choice for this question. Step-by-Step Solution: List authentic defenses: gastric acid and pepsin.Compare with resident flora: sparse in the stomach due to acidity.Select “dense normal flora” as not a major defense at this site. Verification / Alternative check: Clinical observations show achlorhydria or acid suppression increases susceptibility to enteric infections, confirming the protective role of acid rather than flora density in the stomach. Why Other Options Are Wrong: Proteolytic enzymes: present and protective. Low pH: key antimicrobial barrier. All of these: incorrect because two listed are valid defenses. Common Pitfalls: Confusing gastric ecology with colonic ecology; high microbial densities are a feature of the large intestine, not the stomach. Final Answer: Dense normal flora

Question 8

Why annual influenza vaccination? — Choose the best reason people are advised to receive the flu vaccine each year.

Accepted Answer

Introduction / Context: Influenza viruses evolve rapidly by antigenic drift, especially in the hemagglutinin surface glycoprotein that is the principal target of neutralizing antibodies. As a result, immune protection from prior infection or vaccination can diminish when circulating strains acquire new antigenic signatures. This underpins the policy of updating and administering influenza vaccines annually. Given Data / Assumptions: We consider the typical inactivated or live-attenuated influenza vaccines. The immune correlate of protection is mainly antibodies against hemagglutinin. Vaccine composition changes to match predicted circulating strains. Concept / Approach: Yearly vaccination addresses antigenic drift that alters epitopes on hemagglutinin (and neuraminidase). These mutations reduce binding of previously generated antibodies, decreasing neutralization. The vaccine is protein based, not a plain polysaccharide formulation, and modern vaccines are designed to be safe; dose limitations are not due to toxicity but to immunological formulation and manufacturing considerations. Step-by-Step Solution: Identify antigenic drift as the driver of immune escape year to year.Associate drift with hemagglutinin mutations that change antibody epitopes.Select the option that directly states this mechanism. Verification / Alternative check: Global surveillance networks update vaccine strain selections biannually for Northern and Southern Hemispheres, reflecting the need to keep pace with drifted strains. Why Other Options Are Wrong: Polysaccharide vaccine: incorrect; influenza vaccines are not plain polysaccharide vaccines. Toxicity requiring small doses: incorrect; dosing is set for immunogenicity and safety, not because the product is inherently too toxic. None of the above: incorrect because the drift explanation is correct. Common Pitfalls: Confusing antigenic drift with antigenic shift; shift produces pandemics but is less frequent than the seasonal drift that prompts annual vaccination. Final Answer: mutations in the viral hemagglutinin may allow the virus to evade the immune response elicited by previous vaccines

Question 9

Bacterial exotoxins — What is the specific action of the A (active) subunit of diphtheria toxin on host cells?

Accepted Answer

Introduction / Context: Diphtheria toxin, produced by toxigenic Corynebacterium diphtheriae, is a prototypical A-B exotoxin. The B subunit binds to cell surface receptors and facilitates entry, while the A subunit carries catalytic activity that disrupts host processes. Understanding its molecular target explains the profound cytotoxicity and tissue necrosis seen in diphtheria. Given Data / Assumptions: A-B toxins have a binding (B) moiety and an enzymatic (A) moiety. The clinical effect of diphtheria involves inhibition of host cell protein synthesis. We distinguish this action from toxins that raise cAMP or trigger cytokine storms. Concept / Approach: The A subunit catalyzes ADP ribosylation of elongation factor 2 (EF-2), a critical component of the translation machinery. This modification inactivates EF-2 and halts protein synthesis, causing cell death. Other toxins, such as cholera toxin and heat-labile enterotoxin of E. coli, instead alter cAMP through G protein or adenylate cyclase interactions, while superantigens provoke cytokine release without this enzymatic mechanism. Step-by-Step Solution: Identify diphtheria toxin as an A-B toxin where A is enzymatic.Recall that the enzymatic target is EF-2, modified by ADP ribosylation.Conclude that inhibition of protein synthesis is the core mechanism. Verification / Alternative check: Cell culture studies show that expression of the A subunit alone is sufficient to block protein synthesis and kill cells, confirming EF-2 modification as the lethal event. Why Other Options Are Wrong: Binds host cell receptors on heart cells: binding is mediated by the B subunit and is not restricted to heart tissue. Forms cAMP: describes toxins like cholera or pertussis, not diphtheria. Causes macrophage lysis and cytokine release: more aligned with pore-forming toxins or superantigen mechanisms. Common Pitfalls: Confusing ADP ribosylation targets across toxins; the specific target for diphtheria toxin is EF-2, not adenylate cyclase or G proteins. Final Answer: cause ADP ribosylation of a factor involved in protein synthesis

Question 10

Enterotoxins — Cholera toxin is an A–B toxin. After B subunits bind to the host receptor, what does the A subunit do inside the cell?

Accepted Answer

Introduction / Context: Vibrio cholerae causes profuse watery diarrhea via a secreted enterotoxin that hijacks host signaling. Like other A–B toxins, the B subunits mediate binding to specific receptors (GM1 ganglioside), while the A subunit has enzymatic activity that alters intracellular pathways. Knowing the exact target clarifies why cholera leads to massive electrolyte loss without direct mucosal destruction. Given Data / Assumptions: A–B structural organization with receptor binding and enzymatic action. Clinical effect is increased chloride and water secretion from enterocytes. We distinguish among adenylate cyclase, G proteins, and ribosome-targeting actions. Concept / Approach: The A1 fragment of cholera toxin ADP-ribosylates the stimulatory G protein alpha subunit (Gsα), locking it in the GTP-bound active state. This persistent activation continuously stimulates adenylate cyclase, elevating intracellular cAMP. High cAMP drives CFTR chloride channel opening and inhibits sodium absorption, resulting in isotonic fluid loss into the intestinal lumen. The toxin does not ADP-ribosylate adenylate cyclase itself, nor does it target ribosomal RNA, and it acts through cAMP rather than cGMP. Step-by-Step Solution: Identify receptor binding via B subunits to GM1.Recall A subunit activity: ADP ribosylation of Gsα → constitutive activation.Link elevated cAMP to chloride secretion and watery diarrhea. Verification / Alternative check: Pharmacologic inhibitors that block adenylate cyclase or CFTR reduce secretory responses to cholera toxin in model systems, supporting the cAMP mechanism. Why Other Options Are Wrong: ADP ribosylation of adenylate cyclase: incorrect target. rRNA cleavage: describes toxins like Shiga toxin or ricin, not cholera. cGMP stimulation: associated with heat-stable E. coli enterotoxin, not cholera. Common Pitfalls: Confusing the different second messenger pathways; cholera is cAMP-mediated, whereas some E. coli toxins are cGMP-mediated. Final Answer: ADP ribosylation of a G protein that locks it into an active form that stimulates adenylate cyclase to make cAMP

Microorganisms and Disease Questions