**Question:** The drug of choice for chemoprophylaxis in contacts of a patient of pneumonic plague is:

A. Ciprofloxacin
B. Rifampicin
C. Yersinia vaccine
D. Chloramphenicol

**Core Concept:**
Pneumonic plague is a severe and potentially fatal bacterial infection caused by the bacterium Yersinia pestis. Chemoprophylaxis involves the use of antibiotics to prevent the infection in contacts or exposed individuals. In the case of pneumonic plague, the antibiotics should target both the plague bacteria and prevent the development of antibiotic resistance.

**Why the Correct Answer is Right:**
Ciprofloxacin (A) is a fluoroquinolone antibiotic that has broad-spectrum activity against gram-negative bacteria, including Yersinia pestis. It is highly effective against plague bacteria and has low risk of promoting antibiotic resistance.

**Why Each Wrong Option is Incorrect:**
B. Rifampicin (B) is a rifamycin antibiotic that has excellent activity against gram-positive and gram-negative bacteria, including Yersinia pestis. However, it is not as effective as ciprofloxacin against plague bacteria and has higher risk of promoting antibiotic resistance.

C. Yersinia vaccine (C) is primarily used for prevention of primary plague infection in high-risk individuals and populations. It is not suitable for chemoprophylaxis in contacts of a patient with pneumonic plague. The vaccine has limitations in protection against all forms of plague and is not the first-choice option for chemoprophylaxis.

D. Chloramphenicol (D) is an antibiotic that inhibits bacterial protein synthesis. However, it has limited activity against Yersinia pestis and is prone to inducing antibiotic resistance.

**Clinical Pearl:**
In the context of pneumonic plague chemoprophylaxis, it is essential to utilize antibiotics that are highly effective against Yersinia pestis, have low risk of antibiotic resistance development, and are suitable for short-term use in close contacts of a patient with pneumonic plague. Ciprofloxacin meets these criteria, making it the preferred choice for chemoprophylaxis in contacts of a patient with pneumonic plague.

**Why Ciprofloxacin is Right:**
Ciprofloxacin has a high potency against Yersinia pestis and is effective against both plague forms (bubonic and pneumonic). It has a low risk of inducing antibiotic resistance as compared to other antibiotics, making it an optimal choice for chemoprophylaxis in contacts of a patient with pneumonic plague.

**Why Other Options are Incorrect:**
Rifampicin, a rifamycin antibiotic, is effective against Yersinia pestis but has limited activity against pneumonic plague and carries a higher risk of inducing antibiotic resistance. Yersinia vaccine (C) is primarily used for primary prevention and is not suitable for contacts of a patient with pneumonic plague. Chloramphenicol (D