**Question:** Ring sequestrum is seen in -

A. Onychomycosis
B. Psoriasis
C. Epidermolysis bullosa
D. Chronic renal failure

**Core Concept:**
Ring sequestrum, also known as Onycholysis, is a clinical entity characterized by partial or complete detachment of the nail plate from the nail bed. This condition is often associated with systemic diseases or local factors that compromise nail bed circulation. The term "ring" refers to the circular pattern of detachment, which usually starts at the free edge of the nail and moves towards the nail root.

**Why the Correct Answer is Right:**
D. Chronic renal failure is the correct answer because it is a systemic condition that can lead to nail bed ischemia and impaired nail bed circulation. This compromised blood supply can result in ring sequestrum formation. In contrast, the other options are not directly linked to nail bed ischemia or impaired nail bed circulation, making them incorrect answers.

**Why Each Wrong Option is Incorrect:**
A. Onychomycosis (fungal infection of the nail) is a different entity characterized by yellow, thickened, and brittle nails. Ring sequestrum is not seen in onychomycosis, as the infection does not lead to impaired nail bed circulation.
B. Psoriasis is a skin disorder characterized by hyperproliferation of keratinocytes, which is not directly linked to nail bed ischemia and ring sequestrum formation.
C. Epidermolysis bullosa is a group of inherited skin disorders characterized by blistering and fragility of the skin, not directly related to nail bed ischemia and ring sequestrum formation in the context of nail disorders.

**Clinical Pearl:**
Ring sequestration should raise suspicion for underlying systemic diseases like chronic renal failure, and appropriate investigations should be done to identify the root cause. Prompt diagnosis and management of the underlying condition can prevent further nail bed ischemia and prevent the progression of ring sequestration.

**Correct Answer:** D. Chronic renal failure