Synovial biopsy is performed in a suspected case of tuberculosis of the knee. Which histological finding provides the definitive confirmation of the diagnosis?

• A Palisading histiocytes surrounding zones of necrobiotic collagen
• B Fibrinoid necrosis of synovial vessel walls with neutrophilic infiltrate
• C Caseating epithelioid cell granulomas with Langhans giant cells ✓
• D Hyperplastic villi covered by proliferating synoviocytes without necrosis

Explanation

Definitive diagnosis rests on demonstrating caseating granulomatous inflammation composed of epithelioid cells and Langhans giant cells, ideally supported by culture or PCR for Mycobacterium tuberculosis. Acid-fast bacilli are rarely seen on Ziehl-Neelsen staining of tissue sections because the bacillary load in skeletal lesions is scanty, so a negative stain does not exclude the diagnosis. Fibrinoid vasculitis suggests rheumatoid disease, and necrobiotic palisading granulomas suggest a rheumatoid nodule.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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