Differential Diagnosis
- Single most likely diagnosis
- Medication adverse effect: The patient has been taking trimethoprim-sulfamethoxazole for the past year, and the presence of a maculopapular erythematous rash along with eosinophilia (15% eosinophils) suggests an allergic reaction to the medication. This is a common cause of such rashes, especially in the context of long-term antibiotic use.
- Other Likely diagnoses
- Eczema: Although less likely given the acute onset and the patient's lack of history with eczema, it could still be considered, especially if the patient has a personal or family history of atopic diseases. However, the eosinophilia and the context of medication use make this less probable.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): These are severe skin conditions usually triggered by medications, including antibiotics like trimethoprim-sulfamethoxazole. Although the patient's presentation does not currently indicate SJS/TEN (no mention of mucosal involvement, skin detachment, etc.), any skin rash in the context of new medication use should prompt consideration of these life-threatening conditions.
- Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): This is a severe idiosyncratic drug reaction that can occur with various medications, including antibiotics. The presence of eosinophilia, rash, and potential systemic involvement (though not fully evident here) makes DRESS a critical diagnosis not to miss.
- Rare diagnoses
- Staphylococcal or Streptococcal skin infection: While these infections can cause rashes, the description provided (maculopapular erythematous rash over the trunk without specific signs of infection like pus, crust, or significant localized redness and warmth) and the lack of systemic signs of infection (fever, chills) make these less likely. The patient's eosinophilia also points more towards an allergic reaction than a bacterial infection.
- Other allergic or autoimmune conditions: Conditions like pityriasis rosea, psoriasis, or even systemic lupus erythematosus could be considered with a rash, but the context (recent medication use and eosinophilia) makes these less likely without additional supporting evidence.