**Core Concept**
Acute hepatitis refers to the sudden inflammation of the liver, typically caused by viral infections, medications, or other toxins. Early diagnosis is crucial to prevent severe liver damage and complications. The earliest diagnostic marker of acute hepatitis involves assessing liver enzymes and other biochemical parameters.

**Why the Correct Answer is Right**
The earliest diagnostic marker of acute hepatitis is an elevated Alanine Transaminase (ALT) level. ALT is a liver enzyme that leaks into the bloodstream when liver cells are damaged. An increase in ALT levels, often within 24-48 hours of liver injury, is a sensitive indicator of acute hepatitis. This is because ALT is primarily located in the cytosol of hepatocytes, and when these cells are damaged, ALT is released into the bloodstream, allowing for early detection.

**Why Each Wrong Option is Incorrect**
**Option A:** Aspartate Transaminase (AST) levels also rise in acute hepatitis, but AST is less specific for liver damage compared to ALT and can be elevated in other conditions like muscle injury.

**Option B:** Gamma-glutamyl transferase (GGT) levels may be elevated in chronic liver disease or biliary obstruction, making it a less reliable early marker for acute hepatitis.

**Option C:** Alkaline phosphatase (ALP) levels can be elevated in conditions affecting the biliary system, such as obstructive jaundice, and are not specific for acute hepatitis.

**Clinical Pearl / High-Yield Fact**
In cases of suspected acute hepatitis, it's essential to obtain a thorough medical history, including medications, travel history, and potential exposure to infectious agents, as this information can guide targeted diagnostic testing and treatment.

**Correct Answer:** A. Elevated ALT levels.