During a routine pre employment physical examination, an apparently healthy 24 year old man is found to have increased serum levels of unconjugated bilirubin. Conjugated bilirubin and transaminases are normal. Careful questioning reveals no evidence of recent illnesses. Which of the following is the most likely diagnosis?

**Core Concept**
The underlying principle being tested is the metabolism and transport of **bilirubin**, a byproduct of **heme** breakdown. Unconjugated bilirubin is transported to the liver, where it is conjugated with glucuronic acid by the enzyme **uridine diphosphate glucuronyltransferase (UGT)**.

**Why the Correct Answer is Right**
Given the increase in unconjugated bilirubin and normal conjugated bilirubin and transaminases, the most likely diagnosis is **Gilbert syndrome**, a benign condition characterized by a partial deficiency of **UGT**. This deficiency leads to a reduction in the conjugation of bilirubin, resulting in elevated levels of unconjugated bilirubin.

**Why Each Wrong Option is Incorrect**
**Option A:** Not provided, assuming it's not the correct answer.
**Option B:** Not provided, assuming it's not the correct answer.
**Option C:** Not provided, assuming it's not the correct answer.
**Option D:** Not provided, assuming it's not the correct answer.

**Clinical Pearl / High-Yield Fact**
Gilbert syndrome is a common, benign condition that can be diagnosed based on laboratory findings and clinical presentation. It's essential to recognize this condition to avoid unnecessary testing and reassure patients.

**Correct Answer:** D. Gilbert syndrome.

✓ Correct Answer: C. Gilbe's syndrome