An 8-year-old boy who regularly consumes unpasteurized milk presents with chronic abdominal pain and mesenteric lymphadenopathy. The isolated mycobacterium is niacin-negative, nitrate-negative, and grows poorly on routine media. Which first-line antitubercular drug will have no useful activity against this isolate?

• A Isoniazid
• B Rifampicin
• C Ethambutol
• D Pyrazinamide ✓

Explanation

The isolate is Mycobacterium bovis, acquired through ingestion of infected milk. M. bovis is characteristically niacin-negative and nitrate-negative, unlike M. tuberculosis. It is inherently resistant to pyrazinamide because it lacks pyrazinamidase activity, which converts pyrazinamide to its active form, pyrazinoic acid. This is why pyrazinamide is omitted from regimens when M. bovis is confirmed. Isoniazid, rifampicin, and ethambutol all retain activity against M. bovis.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.