Fixed drug eruption due to trimethoprim-sulfamethoxazole
A recurrent pruritic, erythematous, oval macule that appears at the same anatomic site after exposure to trimethoprim-sulfamethoxazole is most consistent with a fixed drug eruption (FDE) to co-trimoxazole. [1,2]
Clinical pattern supporting fixed drug eruption
FDE is characterized by a sharply localized round or oval erythematous patch or plaque that recurs at the same site(s) after re-exposure to the culprit medication. [1,2] FDE commonly presents as a solitary or small number of well-demarcated lesions that may be pruritic. [2] Recurrent FDE episodes often leave post-inflammatory hyperpigmentation that darkens with repeated recurrences. [1]
Most likely causative exposure
Trimethoprim-sulfamethoxazole (and other sulfonamides) are among the commonly implicated drugs in FDE. [3]
Mechanism of recurrence at the same site
FDE recurrence at the identical location is driven by persistent medication-specific memory T cells that remain in the affected skin site. [4]
Key differential diagnosis
Fixed drug eruption should be distinguished from non-drug-related recurrent dermatoses by the presence of same-site recurrence temporally linked to the medication exposure. [1,2]
Management implications of the likely diagnosis
Discontinuation of the implicated medication and avoidance of future exposure to the culprit drug are required to prevent repeated FDE recurrences. [2,3]