Okay, let's tackle this question. The patient is a child with red follicular hyperkeratotic papules on the knees and normal skin in between. Also, there's thickening of palms and soles. The key here is to think about conditions that present with these specific symptoms.

First, hyperkeratotic lesions on the knees and elbows, plus palmoplantar keratoderma, make me think of ichthyosis. But wait, the question mentions follicular papules. Ichthyosis vulgaris usually presents with dry, scaly skin without the follicular pattern. Then there's keratosis pilaris, which does have follicular hyperkeratotic papules, but it's typically on the upper arms and thighs, not knees, and it doesn't cause palmoplantar thickening. Hmm.

Wait, the thickening of palms and soles could point to a more severe condition. Maybe something like keratosis pilaris with palmoplantar keratoderma? But that's not a standard term. Oh, wait! Darier's disease (Darier-White disease) is a genetic disorder characterized by hyperkeratotic papules, often with a follicular pattern, and it can involve the palms and soles. It's an autosomal dominant condition caused by mutations in the ATP2A2 gene. The lesions are usually on the elbows, knees, and face, and they can have a greasy, crusted appearance. The presence of normal skin in between the lesions is a key point here, as Darier's often has this mosaic pattern.

Let me check the other options. Ichthyosis vulgaris is more about scaling without the follicular papules. Psoriasis might have plaques, but not typically follicular papules and palmoplantar thickening. Contact dermatitis is allergic and would present with redness and blisters, not hyperkeratotic papules. So the most likely answer is Darier's disease.

**Core Concept**
This question tests the recognition of **Darier-White disease (keratosis follicularis)**, a **genodermatosis** characterized by **follicular hyperkeratotic papules** and **palmoplantar keratoderma**. It is an **autosomal dominant** disorder caused by mutations in the **ATP2A2 gene**, leading to impaired calcium handling in keratinocytes.

**Why the Correct Answer is Right**
Darier's disease presents with **red, follicular, hyperkeratotic papules** on extensor surfaces (knees, elbows) and **thickened palms/soles**. Lesions are **separated by normal skin** ("mosaic pattern") and often worsen in summer. The **ATP2A2 mutation** disrupts endoplasmic reticulum calcium ATPase, causing abnormal keratinocyte differentiation and acantholysis (loss of cell adhesion). Palmoplantar keratoderma is a hallmark, differentiating it from keratosis pilaris, which lacks palmoplantar involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** *Ichthyosis vulgaris* causes **diffuse scaling** without follicular papules or palmoplantar thickening.
**Option B:** *Psoriasis* presents with **silvery plaques**, not follicular papules, and sp

✓ Correct Answer: C. Pityriasis rubra pilaris