**Core Concept**
Fleeting skin lesions, such as rash or pruritus, are a hallmark of acute viral hepatitis, particularly hepatitis B, due to immune-mediated hypersensitivity reactions. These lesions are transient and often appear during the acute phase of infection, reflecting the body’s immune response to viral antigens.

**Why the Correct Answer is Right**
In viral hepatitis B, the immune system mounts a strong response against hepatitis B surface antigen (HBsAg), leading to immune complex formation and deposition in skin vessels. This results in transient, non-pruritic or pruritic rashes that appear and resolve within days. These lesions are characteristic and often precede or accompany jaundice. The mechanism is immune-mediated, not due to liver failure or systemic infection.

**Why Each Wrong Option is Incorrect**
Option B: Acute cholangitis presents with fever, right upper quadrant pain, and jaundice, but not fleeting skin lesions. The skin manifestations are typically not rashes but rather jaundice or pruritus due to bile duct obstruction.
Option C: Viral hepatitis A causes skin rashes, but they are usually more persistent and associated with gastrointestinal symptoms, not fleeting. The rash in hepatitis A is typically not transient.
Option D: Carcinoma of the head of pancreas causes obstructive jaundice and may lead to pruritus, but not characteristic fleeting skin lesions. The skin findings are due to bile duct obstruction, not immune-mediated rashes.

**Clinical Pearl / High-Yield Fact**
Fleeting skin rashes in hepatitis B are a key clue to acute infection, especially in endemic areas. Remember: **"Hepatitis B rash = immune-mediated, fleeting, often precedes jaundice."**

✓ Correct Answer: A. Viral hepatitis B