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?

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 Understanding that this is a historical reference prevents confusion with current molecular lineage designations (e.g., Lineage 3 or 4) used in contemporary literature.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.