What is the best treatment for a patient with a history of gastrointestinal issues and recent use of Bactrim (sulfamethoxazole/trimethoprim), presenting with a red rash resembling ringworm on their buttocks?

Differential Diagnosis and Management of Ring-Like Rash on Buttocks in Patient with Recent Bactrim Use

Critical First Step: Discontinue Bactrim Immediately

Given the recent Bactrim (trimethoprim-sulfamethoxazole) use and appearance of a rash, this medication must be stopped immediately due to the risk of severe cutaneous adverse reactions including Stevens-Johnson syndrome, toxic epidermal necrolysis, and drug reaction with eosinophilia and systemic symptoms (DRESS). 1

Primary Differential Diagnoses

1. Drug Eruption from Bactrim (Most Likely)

• Bactrim-induced rashes occur in 6-8% of patients and represent the most common adverse reaction to this medication 2, 3
• The FDA label explicitly warns that skin rash may be followed by more severe reactions including Stevens-Johnson syndrome, toxic epidermal necrolysis, and DRESS 1
• Cutaneous reactions typically develop within the first two weeks of therapy 3
• Management approach:
  • Discontinue Bactrim immediately at first appearance of rash 1
  • Monitor closely for progression to severe cutaneous adverse reactions (fever, mucosal involvement, skin detachment, systemic symptoms) 1
  • Consider systemic corticosteroids only if severe reaction develops 1
  • Do NOT rechallenge with sulfonamide-containing medications 4

2. Dermatophyte Infection (Tinea Corporis/Cruris)

If the rash truly resembles ringworm with characteristic annular, scaly, erythematous plaques with central clearing:

• Obtain skin scrapings for KOH preparation and fungal culture to confirm diagnosis 5
• Wood's lamp examination may assist (though dermatophytes typically do not fluoresce) 5
• For confirmed dermatophyte infection on buttocks/groin area:
  • Topical antifungals are first-line for localized disease: miconazole, clotrimazole, or terbinafine cream applied twice daily for 2-4 weeks 5
  • Oral therapy indicated only if: extensive involvement, failed topical therapy, or immunocompromised status 5
  • If oral therapy needed: itraconazole 200 mg daily or terbinafine 250 mg daily 5, 6

Important caveat: In patients with recent Bactrim use presenting with rash, drug eruption must be ruled out first before attributing symptoms to fungal infection, as misdiagnosis could delay recognition of serious drug reactions 4

3. Candidal Intertrigo

Given the buttocks location and patient's GI issues (potential for perianal involvement):

• Candidal infections present as erythematous patches with satellite pustules, not typically ring-like 5
• More common in patients with diabetes, obesity, or immunosuppression 5
• Treatment if confirmed: topical nystatin or azole antifungals (clotrimazole, miconazole) 5
• Oral fluconazole 150 mg single dose may be considered for extensive or recurrent disease 7

Key Clinical Pitfalls to Avoid

1. Never attribute a new rash in a patient on Bactrim to another cause without first stopping the medication - sulfonamide reactions can progress rapidly to life-threatening conditions 1

2. Do not mistake drug eruption for fungal infection - this diagnostic error has been specifically documented in patients receiving sulfonamide or beta-lactam drugs, leading to delayed recognition of serious rickettsial or drug-induced illness 4

3. Avoid restarting any sulfonamide-containing medications - severe reactions including circulatory shock can occur within minutes to hours of rechallenge 1

4. If oral antifungals are eventually needed, be aware of inflammatory flare-up reactions - particularly with zoophilic dermatophyte infections, which can occur 12-24 hours after starting itraconazole or terbinafine 6

Recommended Diagnostic Algorithm

1. Immediately discontinue Bactrim 1
2. Assess for signs of severe drug reaction: fever, mucosal involvement, facial edema, lymphadenopathy, systemic symptoms 1
3. If no severe features present: obtain skin scrapings for KOH prep and fungal culture 5
4. While awaiting culture results: observe without treatment for 48-72 hours to see if rash resolves with Bactrim discontinuation 1
5. If rash persists or worsens despite stopping Bactrim AND fungal studies are positive: initiate topical antifungal therapy 5
6. If severe drug reaction develops: urgent dermatology consultation and consider systemic corticosteroids 1