**Core Concept**
Dubin-Johnson syndrome is a rare genetic disorder characterized by chronic jaundice due to an inability to secrete conjugated bilirubin into the bile. This is caused by mutations in the ATP-binding cassette sub-family C member 2 (ABCC2) gene, leading to the accumulation of conjugated bilirubin in the blood.

**Why the Correct Answer is Right**
The test used to diagnose Dubin-Johnson syndrome is the **crystal retention test**. This test involves administering a load of sulfobromophthalein (BSP) to the patient and then measuring the BSP levels in the urine over time. In individuals with Dubin-Johnson syndrome, the abnormal retention of BSP in the liver results in a prolonged elevation of BSP levels in the urine.

**Why Each Wrong Option is Incorrect**
**Option A:** The **Rome II criteria** are used to diagnose irritable bowel syndrome (IBS), not Dubin-Johnson syndrome.
**Option B:** The **24-hour urine copper test** is used to diagnose Wilson's disease, not Dubin-Johnson syndrome.
**Option C:** The **serum ceruloplasmin test** is used to diagnose Wilson's disease, not Dubin-Johnson syndrome.

**Clinical Pearl / High-Yield Fact**
Dubin-Johnson syndrome is characterized by a distinctive **black liver** on histopathological examination due to the accumulation of epinephrine metabolites.

**Correct Answer: C. Crystal retention test**