## **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, and septicemic forms, each requiring prompt and effective prophylaxis to prevent spread and severe outcomes.

## **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 formulation, which facilitates administration. The use of doxycycline for prophylaxis is supported by guidelines from public health authorities, including the Centers for Disease Control and Prevention (CDC). It is effective in preventing the development of plague when given within a short time frame after exposure.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While streptomycin is an antibiotic that can be used to treat plague, it is not the first choice for chemoprophylaxis due to its parenteral administration route and potential for nephrotoxicity and ototoxicity.
- **Option B:** Ciprofloxacin, a fluoroquinolone, can be used as an alternative for plague prophylaxis, especially in cases where doxycycline is contraindicated. However, it is not considered the drug of choice.
- **Option D:** Sulfadiazine is not typically recommended for plague prophylaxis as it is less effective against *Yersinia pestis* compared to doxycycline.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that doxycycline is the preferred agent for chemoprophylaxis of plague, especially in high-risk exposures. For individuals traveling to areas where plague is endemic, doxycycline can be considered for prophylaxis if they will be at high risk of exposure (e.g., veterinarians, laboratory workers).

## **Correct Answer:** . Doxycycline