**Core Concept**
The question is testing the interaction between antitubercular therapy and antiretroviral therapy in HIV patients. The underlying principle is the potential for drug-drug interactions and their impact on treatment efficacy and safety.

**Why the Correct Answer is Right**
Streptomycin, an aminoglycoside antibiotic, is a first-line antitubercular agent. However, it is contraindicated in HIV patients due to its potential to cause nephrotoxicity and ototoxicity, which can be exacerbated by the concurrent use of antiretroviral medications such as tenofovir. The combination of these drugs can increase the risk of renal impairment and hearing loss.

**Why Each Wrong Option is Incorrect**
* **Option A:** Isoniazid is a first-line antitubercular agent and is generally safe in HIV patients. It is often used in combination with other medications to treat TB in individuals with HIV.
* **Option B:** Rifampicin is a potent inducer of cytochrome P450 enzymes and can interact with various antiretroviral medications, but it is not typically contraindicated in HIV patients.
* **Option C:** Ethambutol is a second-line antitubercular agent and is generally safe in HIV patients. It can cause optic neuritis, but this is not a contraindication for use in HIV patients.

**Clinical Pearl / High-Yield Fact**
When managing TB in HIV patients, it's essential to consider potential drug-drug interactions and monitor for signs of nephrotoxicity and ototoxicity, particularly when using aminoglycosides like streptomycin.

**Correct Answer: C. Ethambutol is generally safe in HIV patients and is not typically contraindicated.