Differential Diagnosis for the 18-year-old Male with a New Rash
- Single most likely diagnosis
- Guttate Psoriasis: The patient's presentation of a solitary, scaly, pink plaque followed by the development of several smaller, discrete pink plaques and patches on the trunk, oriented along skin cleavage lines, is highly suggestive of guttate psoriasis. The mild pruritus and absence of facial involvement also support this diagnosis.
- Other Likely diagnoses
- Pityriasis Rosea: This condition is characterized by a herald patch followed by a secondary eruption of smaller, oval-shaped patches, often distributed along skin cleavage lines. The patient's symptoms and the timing of the rash are consistent with pityriasis rosea.
- Tinea Corporis: A fungal infection of the skin, which can present with scaly, pink plaques and patches. However, the distribution along skin cleavage lines and the absence of a clear border are less typical for tinea corporis.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Lyme Disease (Erythema Migrans): Although the patient denies being sexually active and the rash does not have a typical "bull's-eye" appearance, erythema migrans can present with a solitary, expanding plaque. It is essential to consider Lyme disease, especially if the patient has been exposed to ticks.
- Syphilis (Secondary Syphilis): The patient's denial of sexual activity does not entirely rule out syphilis, as he may not be aware of his exposure or may not want to disclose it. A rash on the trunk, although not typically the primary presentation, can occur in secondary syphilis.
- Rare diagnoses
- Pityriasis Lichenoides: A rare skin condition characterized by small, scaly papules that can resemble guttate psoriasis or pityriasis rosea. The diagnosis is often made after excluding other conditions.
- Nummular Dermatitis: A type of eczema that can present with coin-shaped patches, but it is less likely given the patient's age and the distribution of the rash along skin cleavage lines.