Differential Diagnosis for a Non-Painful Red Raised Rash Localized to the Knee
Single Most Likely Diagnosis
- Keratosis Pilaris: A common skin condition characterized by small, rough, red or brown bumps on the skin, often appearing in clusters. The knee is a common location, and the size of the lesions (2-4 mm) matches the typical presentation of keratosis pilaris.
Other Likely Diagnoses
- Eczema (Atopic Dermatitis): A chronic skin condition that can cause red, raised, and itchy patches, which may appear in clusters. Although eczema is often itchy, some cases can be non-painful.
- Folliculitis: An inflammation of the hair follicles, which can cause red, raised bumps, often in clusters. The knee is a common location, especially if the skin is irritated or infected.
- Pityriasis Rosea: A skin condition characterized by small, raised, red patches, often appearing in clusters. Although it typically starts with a single "herald patch," subsequent lesions can appear in clusters.
Do Not Miss Diagnoses
- Erythema Multiforme: A skin condition that can cause red, raised, target-shaped lesions, which can appear in clusters. Although it's often associated with other symptoms, such as fever or mucous membrane involvement, it's essential to consider this diagnosis due to its potential severity.
- Lymphoma Cutis: A rare skin manifestation of lymphoma, which can cause red, raised lesions, often in clusters. Although unlikely, missing this diagnosis could have severe consequences.
Rare Diagnoses
- Granuloma Annulare: A rare skin condition characterized by small, raised, red or purple bumps, often in a ring-like pattern. Although it can appear on the knee, it's less common than other diagnoses on this list.
- Lichen Planus: A chronic skin condition that can cause small, raised, red or purple bumps, often with a distinctive ring-like pattern. Although it can appear on the knee, it's relatively rare and often associated with other symptoms, such as itching or mucous membrane involvement.