Differential Diagnosis for a Non-Painful Red Raised Rash Localized to the Knee
Single Most Likely Diagnosis
- Keratosis Pilaris: This condition is characterized by small, rough, sandpaper-like bumps on the skin, often on the knees, elbows, or buttocks. The description of small, raised, red lesions in clusters on the knee fits well with keratosis pilaris, especially given its common occurrence on the knees.
Other Likely Diagnoses
- Eczema/Dermatitis: Atopic dermatitis or contact dermatitis could present with red, raised lesions, especially if there's an irritant or allergen involved that the patient comes into contact with around the knee area.
- Folliculitis: An inflammation of the hair follicles, which can appear as small, red, raised bumps, often in clusters. This could be due to bacterial, fungal, or viral infections.
- Pityriasis Rosea: Although it typically starts with a herald patch, the subsequent rash can appear as small, raised, red lesions in a Christmas tree distribution, which might be localized to the knee area in some cases.
Do Not Miss Diagnoses
- Lyme Disease: The rash of Lyme disease (erythema migrans) can sometimes appear as multiple smaller lesions rather than a single large one. Although it's typically associated with a tick bite and can be painful, it's crucial not to miss this diagnosis due to its potential long-term consequences if left untreated.
- Cutaneous Lupus: Discoid lupus erythematosus can present with red, raised lesions, and while it's less common to be localized solely to the knee, systemic lupus erythematosus can have varied presentations, making it a diagnosis not to be missed due to its potential systemic implications.
Rare Diagnoses
- Granuloma Annulare: A benign condition that can appear as small, raised, red bumps in a ring-like pattern, though it's less common for it to be exclusively localized to the knee.
- Sarcoidosis: A condition that can cause skin lesions, among other systemic symptoms. The skin lesions can be red, raised, and appear in various patterns, including on the knees, though this would be an uncommon presentation without other systemic symptoms.