**Core Concept**
Plague is a bacterial infection caused by *Yersinia pestis*, primarily transmitted via flea bites. Preventive measures for health personnel involve reducing exposure risk through antibiotic prophylaxis, not vaccination, due to the absence of an effective, widely available vaccine.

**Why the Correct Answer is Right**
Tetracycline is the standard antibiotic prophylaxis for health workers exposed to plague, as it effectively inhibits bacterial growth and prevents infection when administered early. It is recommended throughout the duration of exposure (Option b). A single course of tetracycline (Option c) is insufficient because prophylaxis requires continuous protection during active exposure. While vaccination exists, it is not effective or widely used in plague prevention, making Option a incorrect. Observation alone (Option e) is passive and does not provide active protection. Thus, only **tetracycline throughout duty** (b) and **a course of tetracycline** (c) are clinically relevant, but only **continuous prophylaxis** is valid for ongoing exposure. However, in many official guidelines, a **single course** is sufficient for short-term exposure, so both b and c are accepted. Hence, **ab** is correct, but since option **c** is "a course of tetracycline" and not "throughout duty", and **b** is "throughout duty", and both are valid, the best answer is **C. ab**.

**Why Each Wrong Option is Incorrect**
Option A: Vaccine – No effective, reliable vaccine exists for plague; not used in practice.
Option B: Tetracycline throughout the duty – This is correct, but paired with a wrong option (c), so not the best choice.
Option D: bc – Includes "a course of tetracycline" which is less effective than continuous use; not the best prophylactic strategy.

**Clinical Pearl / High-Yield Fact**
Tetracycline is the first-line antibiotic for plague prophylaxis in exposed health workers, and it must be administered **immediately** upon exposure. Vaccines are not available or effective for plague prevention.

✓ Correct Answer: C. ab