**Core Concept**
The McEwan sign, also known as the "halo sign," is a clinical indicator of liver disease, particularly cirrhosis. It is characterized by a yellowish discoloration of the skin around the eyes, giving the appearance of a halo. This sign is a manifestation of the body's attempt to eliminate excess bilirubin, a byproduct of liver dysfunction.

**Why the Correct Answer is Right**
The McEwan sign is a classic sign of liver disease, and among the options provided, alcoholism is the most common cause of cirrhosis. Chronic alcohol consumption leads to liver damage, resulting in the accumulation of bilirubin, which is then deposited in the skin around the eyes, causing the characteristic yellowish discoloration. This process is facilitated by the impaired liver function, which fails to effectively conjugate and excrete bilirubin.

**Why Each Wrong Option is Incorrect**
**Option A:** Cyanide poisoning primarily affects the respiratory and cardiovascular systems, causing rapid death due to asphyxiation and cardiac arrest. It does not typically cause liver dysfunction or the McEwan sign.

**Option C:** Lead poisoning primarily affects the hematopoietic and nervous systems, causing anemia and neurological symptoms. While lead can cause liver damage, it is not the primary cause of the McEwan sign.

**Option D:** Arsenic poisoning primarily affects the gastrointestinal and cardiovascular systems, causing vomiting, diarrhea, and cardiac arrhythmias. While arsenic can cause liver damage, it is not the primary cause of the McEwan sign.

**Clinical Pearl / High-Yield Fact**
The McEwan sign is a classic clinical indicator of liver disease and should be considered in patients with a history of chronic alcohol consumption or other risk factors for liver disease. It is essential to perform a thorough medical history and physical examination to identify patients at risk of liver disease.

**✓ Correct Answer: B. Alcoholism**