What is the most likely cause of a recurrent pruritic erythematous oval macule on the left thigh in a 28-year-old woman after treatment of a urinary tract infection with trimethoprim‑sulfamethoxazole (Bactrim)?

Drug-Induced Erythema Multiforme (Fixed Drug Eruption)

This recurrent pruritic erythematous oval macule appearing in the same location after Bactrim (trimethoprim-sulfamethoxazole) treatment is most likely a fixed drug eruption, a localized form of drug hypersensitivity reaction.

Clinical Reasoning

The key diagnostic features pointing to fixed drug eruption are:

• Recurrence in the identical anatomical location (left thigh, both times)
• Temporal relationship to Bactrim exposure (appears after UTI treatment)
• Classic morphology (pruritic erythematous oval macule)
• History of identical lesion with previous UTI treatment (suggesting previous Bactrim exposure)

Understanding the Mechanism

Trimethoprim-sulfamethoxazole is well-documented to cause cutaneous hypersensitivity reactions. The FDA label explicitly warns that "BACTRIM should be discontinued at the first appearance of skin rash or any sign of a serious adverse reaction" as skin rash may progress to severe reactions including Stevens-Johnson syndrome, toxic epidermal necrolysis, or erythema multiforme 1.

Fixed drug eruptions are a specific type of drug hypersensitivity where:

• Lesions recur at the exact same site with re-exposure to the offending drug
• The sulfonamide component (sulfamethoxazole) is a common culprit
• Lesions typically appear as round or oval erythematous patches or plaques
• Post-inflammatory hyperpigmentation often remains after resolution

Evidence Supporting This Diagnosis

Multiple case reports confirm trimethoprim-sulfamethoxazole as a causative agent for erythema multiforme and drug eruptions:

• A 29-year-old woman developed erythema multiforme with itching, rash, and vesicles after trimethoprim-sulfamethoxazole exposure 2
• A 91-year-old male developed bullous erythema multiforme after TMP-SMX treatment 3
• A 58-year-old woman developed a drug exanthema with strong pruritus following TMP-SMX therapy, with lymphocyte transformation testing confirming sensitization to sulfamethoxazole 4

Critical Management Points

The patient should be advised to avoid all sulfonamide-containing medications permanently, as:

1. Re-exposure carries risk of more severe reactions
2. The FDA warns that reactions can progress from simple rash to life-threatening conditions 1
3. Fixed drug eruptions typically worsen with repeated exposures

Alternative UTI Treatment

For future UTI episodes, the patient should receive:

• Nitrofurantoin (first-line alternative)
• Fluoroquinolones (ciprofloxacin, levofloxacin) if nitrofurantoin contraindicated
• Fosfomycin (single-dose option)
• Beta-lactams (amoxicillin-clavulanate, cephalosporins)

Document this allergy prominently in the medical record to prevent future exposures.

Common Pitfall to Avoid

Do not dismiss this as a simple "rash" or continue Bactrim therapy. The recurrence in the identical location is pathognomonic for fixed drug eruption and indicates established drug hypersensitivity. While this patient's lesion is currently localized and mild, the FDA explicitly warns that continued exposure or re-challenge can lead to severe cutaneous adverse reactions, including potentially fatal conditions 1.