## **Core Concept**
The question tests knowledge on the chemoprophylaxis of plague, specifically focusing on the drug of choice (DOC) for preventing the disease. Plague, caused by *Yersinia pestis*, can manifest in bubonic, pneumonic, or septicemic forms, each requiring different approaches to prevention and treatment.

## **Why the Correct Answer is Right**
Doxycycline is the recommended drug for chemoprophylaxis of plague due to its efficacy against *Yersinia pestis* and its oral administration route, which facilitates ease of use in outbreak scenarios. The antibiotic works by inhibiting protein synthesis in bacteria, thereby preventing their growth and proliferation. For individuals exposed to pneumonic plague, doxycycline is particularly crucial as it can prevent the development of the disease if started promptly.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While streptomycin is effective against plague, it is primarily used for treatment rather than chemoprophylaxis due to its injectable administration route and potential for nephrotoxicity and ototoxicity.
- **Option B:** Ciprofloxacin can be used as an alternative for plague prophylaxis, especially in cases where doxycycline is contraindicated. However, it is not the first-line recommendation due to concerns about resistance development and its slightly broader spectrum of activity which might not be necessary for *Yersinia pestis*.
- **Option D:** Gentamicin, an aminoglycoside antibiotic, can be used to treat plague but is not preferred for chemoprophylaxis due to its potential for nephrotoxicity and ototoxicity, as well as its requirement for parenteral administration.

## **Clinical Pearl / High-Yield Fact**
A critical point to remember is that doxycycline is the preferred agent for chemoprophylaxis of plague, especially in close contacts of patients with pneumonic plague. Its use can significantly reduce the risk of developing the disease. Additionally, chemoprophylaxis is typically recommended for individuals who have had close contact with someone with pneumonic plague.

## **Correct Answer:** C. Doxycycline.