**Core Concept**
Macewan's sign is a clinical finding associated with chronic alcoholism, characterized by the presence of a blue or purple discoloration of the skin over the upper chest and shoulders due to impaired microcirculation and capillary dilation. It reflects the toxic effect of ethanol on the vascular system and is a hallmark of long-standing alcohol abuse.

**Why the Correct Answer is Right**
In alcoholism, chronic ethanol consumption leads to vasodilation and impaired vascular tone, particularly in the dermal capillaries. This results in visible bluish discoloration of the skin, especially over the upper chest and shoulders, known as Macewan’s sign. The mechanism involves ethanol-induced endothelial dysfunction and increased capillary permeability. This sign is not specific to alcoholism but is classically linked to it in forensic and clinical settings.

**Why Each Wrong Option is Incorrect**
Option A: Cyanide poisoning presents with cherry-red skin, not blue discoloration, and causes rapid respiratory failure and metabolic acidosis—Macewan’s sign is not seen here.
Option C: Lead poisoning causes abdominal pain, encephalopathy, and "lead lines" on X-ray, not skin discoloration.
Option D: Arsenic poisoning manifests with gastrointestinal symptoms, neurological deficits, and peripheral neuropathy, with no association to Macewan’s sign.

**Clinical Pearl / High-Yield Fact**
Macewan’s sign is a key physical finding in chronic alcoholism, especially in cases of suspected alcohol-related disease. Its presence should prompt a thorough evaluation of alcohol use and potential withdrawal or liver dysfunction.

✓ Correct Answer: B. Alcoholism