A 40-year-old woman presents with palpable purpura on both lower limbs of three days' duration. Skin biopsy shows leucocytoclastic vasculitis on routine histology. For maximum yield of immunoreactants on direct immunofluorescence, the biopsy ideally should have been taken:

• A From the oldest available lesion, since deposits accumulate over time
• B After one week, once the lesion is fully evolved
• C Within 24 to 48 hours of lesion onset ✓
• D Timing does not matter because immune deposits persist indefinitely

Explanation

In cutaneous small-vessel vasculitis, granular IgM and B3 deposits in vessel walls are best demonstrated in lesions less than about 24 to 48 hours old. With time, immunoglobulins are degraded by neutrophil enzymes and washed out, so older lesions show fibrinogen alone or a completely negative study despite definite vasculitis on histology. This is why an early lesion, not the oldest one, gives the highest diagnostic yield.

Reference: Bolognia, Schaffer and Cerroni, Dermatology, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.