**Core Concept**
Wilson's disease is a rare genetic disorder characterized by excessive accumulation of copper in the body, particularly in the liver, brain, and other organs. This occurs due to mutations in the ATP7B gene, which encodes a copper-transporting ATPase responsible for regulating copper levels in the body.

**Why the Correct Answer is Right**
The Kayser-Fleischer (KF) ring is a pathognomonic sign of Wilson's disease, representing a copper deposit in the Descemet membrane of the cornea. However, its presence is not as common as previously thought. Studies have shown that KF rings are present in approximately 70-80% of patients with Wilson's disease, making option D incorrect. The correct statement should reflect the higher prevalence of KF rings in Wilson's disease patients.

**Why Each Wrong Option is Incorrect**
**Option A:** ATP7B mutations indeed lead to impairment in copper metabolism, causing excessive copper accumulation in the body. This is a fundamental aspect of Wilson's disease pathophysiology.
**Option B:** The most specific screening test for Wilson's disease is indeed the urinary excretion of copper, particularly after a penicillamine challenge. This test measures the body's ability to excrete copper in the urine.
**Option C:** Liver biopsy is considered the gold standard test for diagnosing Wilson's disease, as it allows for the direct measurement of copper levels in the liver tissue.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Wilson's disease is a treatable condition, and early diagnosis is crucial to prevent long-term complications. A high index of suspicion, coupled with a thorough clinical evaluation and laboratory tests, is necessary to diagnose Wilson's disease accurately.

**✓ Correct Answer: D. KF ring is present in upto 30% cases**