A 20-year-old male presents with a rash on the scalp, (extensor) elbows, knees and umbilicus. The rash is sharply demarcated erythematous plaques with silvery scale in those locations; there is no central clearing. The soles of the feet are normal. What is the most likely diagnosis?

**Core Concept**
The patient's presentation is suggestive of a dermatological condition characterized by sharply demarcated erythematous plaques with silvery scale, primarily affecting extensor surfaces and the umbilicus. This condition is a classic example of a chronic inflammatory skin disease.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with psoriasis, a chronic autoimmune condition resulting from an abnormal immune response to self-antigens. The silvery scale is due to the accumulation of keratinocytes, which are produced in excess due to the abnormal proliferation of skin cells. The involvement of the elbows, knees, and scalp, along with the sparing of the soles, is a classic distribution pattern for psoriasis. The lack of central clearing is also a distinguishing feature of psoriasis, as opposed to other skin conditions like eczema or dermatitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Seborrheic dermatitis typically presents with a greasy, yellowish scale, often affecting the scalp and face, and is not characterized by sharply demarcated erythematous plaques.
**Option B:** Eczema (atopic dermatitis) often presents with pruritic, vesicular, or erythematous lesions, and may have a more widespread distribution, including the soles.
**Option C:** Dermatitis herpetiformis presents with intensely itchy, vesicular lesions, often on the elbows, knees, and buttocks, but typically has a more widespread distribution and is associated with gluten sensitivity.

**Clinical Pearl / High-Yield Fact**
Psoriasis is a systemic condition, not just a skin disease, and patients with severe psoriasis are at increased risk of developing psoriatic arthritis, cardiovascular disease, and depression.

**Correct Answer:** C.

✓ Correct Answer: C. Psoriasis