Isoniazid (INH), used to treat tuberculosis, has a significant long-term adverse effect on which organ system?

Correct answer: C. Liver (hepatotoxicity — INH-induced drug-induced liver injury)

• A. Peripheral neuropathy (pyridoxine/B6 depletion) is a side effect, but the most significant long-term harm is liver damage.
• B. Bone marrow suppression is not a recognized primary adverse effect of isoniazid.
• C. Correct. The major long-term adverse effect of isoniazid is HEPATOTOXICITY (drug-induced liver injury/DILI). INH is metabolized by the liver to hepatotoxic metabolites (hydrazine). Patients on long-term INH therapy require periodic liver function monitoring. INH also causes peripheral neuropathy (via B6 depletion), but the most clinically significant long-term concern is hepatotoxicity.
• D. While some drugs cause nephrotoxicity, isoniazid's primary long-term concern is hepatic — not renal.