A granuloma is examined histologically and shows a central zone of necrosis with a soft, cheese-like appearance. Which aetiology is most likely?

Correct answer: D. Mycobacterium tuberculosis

• A. Enzymatic destruction of fat by released lipases gives FAT necrosis with chalky-white saponification, classically in acute pancreatitis.
• B. Ischaemia and hypoxia produce COAGULATIVE necrosis, in which tissue architecture and cell outlines are PRESERVED as ghost shapes — the pattern seen in myocardial and renal infarction. The exception is the brain, which undergoes liquefactive necrosis.
• C. Autolysis and heterolysis by hydrolytic enzymes give LIQUEFACTIVE necrosis, a viscous liquid mass — characteristic of bacterial abscesses and of cerebral infarction.
• D. CORRECT. CASEOUS necrosis — grossly soft, friable and cheese-like; microscopically an amorphous, granular eosinophilic debris in which cell outlines are completely lost — is the signature of MYCOBACTERIUM TUBERCULOSIS. The high lipid content of the mycobacterial cell wall contributes to the appearance, and the surrounding rim of epithelioid histiocytes, Langhans giant cells and lymphocytes completes the granuloma. Other causes of caseating granulomas include histoplasmosis and other systemic fungi.

Note: Oral relevance: an oral tuberculous ulcer is usually a painful, ragged ulcer with undermined edges on the DORSUM OF THE TONGUE, most often secondary to pulmonary disease from infected sputum. Biopsy shows caseating granulomas; acid-fast staining and culture confirm it.