Bullous pemphigoid
B. Perform skin biopsy with direct immunofluorescence study
The key to the correct diagnosis was the pattern of tense bullae with clear exudate and associated severe pruritus, typical for bullous pemphigoid. Recognizing bullous pemphigoid features helps avoid unnecessary antimicrobial treatment and diagnostic interventions. Biopsy with direct immunofluorescence (DIF) revealed a linear pattern of IgG along the dermal-epidermal junction, consistent with the diagnosis of bullous pemphigoid.1 Aspiration of the knee to investigate for prosthetic joint infection (choice A) is not indicated in this case, as the patient had no symptoms concerning for an intra-articular infection, such as pain or warmth. A diagnosis of allergic and irritant contact dermatitis (ACD) may be confirmed with skin patch testing (choice C); however, in this patient, the bullae were larger than would be expected for ACD, and symptoms would have appeared earlier in the postoperative course and should have resolved with oral corticosteroids. Further antibiotic treatment for bullous impetigo (choice D) would not be indicated for this patient, as the clinical appearance was not consistent with impetigo (lack of honey-crusted lesions) and there was no improvement with either topical iodine or oral cephalexin.