Dermatitis herpetiformis — SCE Dermatology MCQ

Correct answer: A — Direct immunofluorescence from uninvolved perilesional skin

The correct answer is A, direct immunofluorescence from uninvolved perilesional skin. Dermatitis herpetiformis is the cutaneous manifestation of coeliac disease, explaining the grouped excoriated papulovesicles on extensor surfaces and buttocks, intense pruritus, and iron deficiency with bloating from small bowel enteropathy. Routine histology of an excoriated or crusted lesion is frequently non-diagnostic because the neutrophilic papillary tip abscesses and subepidermal vesicle are destroyed by scratching. The diagnostic gold standard is direct immunofluorescence of clinically normal skin adjacent to a lesion, which demonstrates granular IgA deposition at the dermal papillary tips, a finding preserved even when routine biopsy is uninterpretable. Why the other options are wrong: C. Bacterial culture from an excoriated vesicle: excoriated lesions are commonly secondarily infected, but culture identifies superinfection only and does not establish the underlying autoimmune diagnosis. D. Serum pemphigoid antibodies as the first test: this targets bullous pemphigoid, which presents with tense bullae in older patients rather than grouped excoriated extensor papulovesicles, and does not explain the coeliac features. E. Patch testing to topical medicaments: this investigates allergic contact dermatitis, which does not produce grouped vesicles with papillary dermal IgA or link to enteropathy. B. Repeat biopsy from a crusted excoriation: repeating histology from another excoriated or crusted area will again show non-specific inflammatory change because the primary vesicle architecture is destroyed by scratching, regardless of site chosen. Key point: When lesional histology in suspected dermatitis herpetiformis is non-specific due to excoriation, biopsy uninvolved perilesional skin for direct immunofluorescence to demonstrate granular IgA at the dermal papillary tips.

Reference: DermNet NZ (authored by UK dermatologists, reviewed in line with BAD practice), Dermatitis Herpetiformis: Features, Diagnosis, and Treatment, 2023, https://dermnetnz.org/topics/dermatitis-herpetiformis