**Question:** What is deposited in Kayser-Fleischer ring -

A. Bilirubin
B. Lipids
C. Plasma proteins
D. Hemosiderin

**Core Concept:**
The Kayser-Fleischer (KF) ring is a characteristic finding in the cornea due to the accumulation of copper-rich pigment. It is typically observed in patients with Wilson's disease, a genetic disorder of copper homeostasis. Wilson's disease is caused by mutations in the ATP7B gene, leading to impaired liver function and copper accumulation in the body.

**Why the Correct Answer is Right:**
The Kayser-Fleischer ring results from the deposition of copper-containing pigments, specifically ceruloplasmin-bound copper (Cu-ceruloplasmin), in the corneal epithelium. In Wilson's disease, copper is accumulated due to impaired liver function and copper-sequestering proteins like ceruloplasmin are unable to clear the excess copper effectively.

**Why Each Wrong Option is Incorrect:**
A. Bilirubin: Bilirubin is a product of heme catabolism and is primarily excreted into the bile. It does not accumulate in the cornea forming a ring.
B. Lipids: Lipids are not the primary component deposited in the Kayser-Fleischer ring. The ring is primarily formed due to copper deposition.
C. Plasma proteins: While plasma proteins are involved in copper homeostasis, the specific copper-bound protein, ceruloplasmin, is responsible for copper deposition in the Kayser-Fleischer ring, making option C incorrect as well.
D. Hemosiderin: Hemosiderin is iron-rich pigment and is not associated with the Kayser-Fleischer ring.

**Clinical Pearl:**
Patients with Wilson's disease should be evaluated for this clinical sign, as it may help in the diagnosis of the condition. Other conditions that can present with Kayser-Fleischer ring include copper intoxication, copper-induced liver cirrhosis, and copper-induced pulmonary fibrosis.

**Correct Answer:**
D. Hemosiderin: Although hemosiderin is an iron-rich pigment, it is associated with conditions like hemosiderosis, a condition involving iron overload, rather than copper accumulation. In Wilson's disease, copper is deposited in the corneal epithelium, forming the Kayser-Fleischer ring. Hemosiderin is not involved in this process.