**Core Concept**
Brucellosis is a zoonotic infection caused by the bacterium Brucella, which is primarily transmitted through consumption of contaminated dairy products or direct contact with infected animals. Effective treatment of brucellosis requires antibiotics that can penetrate into the reticuloendothelial system, where the bacteria reside.

**Why the Correct Answer is Right**
The drug of choice for treating brucellosis is a combination of doxycycline and rifampicin. This combination is effective because doxycycline inhibits protein synthesis in Brucella, while rifampicin interferes with DNA synthesis, ultimately leading to bacterial death. The combination also allows for a shorter treatment duration compared to monotherapy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptomycin is an aminoglycoside antibiotic that is effective against a wide range of bacterial infections, but it is not the drug of choice for brucellosis due to its poor penetration into the reticuloendothelial system.
* **Option B:** Ciprofloxacin is a fluoroquinolone antibiotic that is effective against many bacterial infections, but it is not typically used to treat brucellosis due to concerns about resistance and inadequate penetration into the reticuloendothelial system.
* **Option C:** Ampicillin is a beta-lactam antibiotic that is effective against many bacterial infections, but it is not the drug of choice for brucellosis due to concerns about resistance and inadequate penetration into the reticuloendothelial system.

**Clinical Pearl / High-Yield Fact**
It's essential to note that brucellosis can relapse if treatment is not completed, and relapse can occur months or even years after the initial treatment. Therefore, it's crucial to ensure that patients with brucellosis complete the full course of antibiotics as prescribed.

**Correct Answer: D. Doxycycline and rifampicin.**