Differential Diagnosis for Pruritic Erythematous Papular Rash
Given the patient's presentation of a new pruritic erythematous papular rash, low risk for sexually transmitted diseases (STDs) due to having only one sexual partner in the last year, and denial of any penile discharge or pain, the following differential diagnoses are considered:
- Single Most Likely Diagnosis
- Contact Dermatitis: This is a common condition that can cause a pruritic erythematous papular rash. It occurs when the skin comes into contact with an irritant or allergen, leading to an immune response. Given the patient's low risk for STDs and the nature of the rash, contact dermatitis is a plausible explanation.
- Other Likely Diagnoses
- Scabies: Although the patient reports a low risk for STDs, scabies is highly contagious and can be spread through close contact, not necessarily sexual. It causes a pruritic rash and can present with erythematous papules.
- Eczema (Atopic Dermatitis): This chronic condition can cause pruritic erythematous rashes. While it's more common in children, adults can also develop eczema, especially if they have a history of atopy.
- Psoriasis: An autoimmune condition that can cause erythematous papular rashes. It's less likely given the acute presentation but should be considered, especially if the patient has a family history or other symptoms suggestive of psoriasis.
- Do Not Miss Diagnoses
- Syphilis (Secondary): Despite the patient's reported low risk, syphilis is a critical diagnosis not to miss due to its potential for severe complications if left untreated. Secondary syphilis can cause a wide range of skin manifestations, including a pruritic erythematous papular rash.
- HIV-Related Dermatologic Conditions: Certain skin conditions are associated with HIV infection, such as seborrheic dermatitis, psoriasis, and drug reactions. Given the importance of early diagnosis and treatment of HIV, considering HIV-related conditions is crucial, even in low-risk patients.
- Rare Diagnoses
- Pityriasis Rosea: A condition that causes a rash, often preceded by a herald patch, followed by a secondary eruption of oval-shaped, scaling patches. It's less common and typically seen in younger individuals but could be considered in the differential diagnosis.
- Lichen Planus: An inflammatory condition that affects mucous membranes inside the mouth and can also cause a purplish, itchy, flat rash on the skin. It's less likely given the description but is a rare condition that could fit some aspects of the patient's presentation.
Each of these diagnoses should be considered in the context of the patient's full clinical presentation, including history, physical examination findings, and the results of the STD screening and any additional diagnostic tests that may be warranted.