**Core Concept**
The question pertains to the hepatotoxicity of antitubercular drugs, a critical consideration in the management of tuberculosis (TB). Hepatotoxicity refers to liver damage caused by certain medications, including some used to treat TB. The liver plays a vital role in metabolizing these drugs, and hepatotoxicity can be a significant side effect.

**Why the Correct Answer is Right**
Streptomycin, an aminoglycoside antibiotic, is primarily excreted through the kidneys and does not undergo significant hepatic metabolism. Unlike other antitubercular drugs, streptomycin is not hepatotoxic, making it a safer option for patients with pre-existing liver disease or those at risk of liver damage. This is because aminoglycosides primarily affect the auditory and vestibular systems and the kidneys, rather than the liver.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid is a first-line antitubercular agent that undergoes hepatic metabolism and is known to cause hepatotoxicity in a small percentage of patients. Isoniazid-induced liver damage can be severe and may require discontinuation of the drug.

**Option B:** Rifampicin is another first-line antitubercular agent that can cause hepatotoxicity, particularly when used in combination with isoniazid. Rifampicin-induced liver damage is often mild but can be severe in rare cases.

**Option C:** Ethionamide is a second-line antitubercular agent that is also known to cause hepatotoxicity. Ethionamide-induced liver damage can be severe and may require discontinuation of the drug.

**Clinical Pearl / High-Yield Fact**
When treating TB, it is essential to monitor liver function tests regularly, particularly in patients taking first-line antitubercular agents like isoniazid and rifampicin. Early identification of hepatotoxicity can prevent severe liver damage and improve patient outcomes.

**✓ Correct Answer: D. Streptomycin. Streptomycin is not hepatotoxic.**