A patient is taking Isoniazid for tuberculosis. What is a serious potential side effect?

Isoniazid (INH) carries a black box warning for severe and potentially fatal hepatitis. Transient asymptomatic elevation of liver enzymes occurs in 15-20% of patients. Clinically apparent hepatitis occurs in 0.5-1% of recipients and is most common in patients over 35, alcohol users, and those with pre-existing liver disease. Peripheral neuropathy (option d) is also a known side effect of INH (due to pyridoxine/vitamin B6 depletion), but hepatitis is the most serious concern requiring monitoring. Regular liver function tests are recommended during INH therapy. Both hepatotoxicity and peripheral neuropathy are correct answers, but hepatitis represents the most dangerous (potentially fatal) side effect that requires the most vigilance.