## **Core Concept**
Dubin-Johnson syndrome is a rare genetic disorder characterized by an inability to secrete conjugated bilirubin into the bile, leading to chronic jaundice. This condition is associated with mutations in the ABCC2 gene, which encodes for a canalicular multidrug resistance-associated protein 2 (MRP2) involved in bilirubin transport. The diagnosis involves specific tests that highlight the liver's inability to transport bilirubin.

## **Why the Correct Answer is Right**
The correct answer, **C. Hepatic uptake of radioactive material (e.g., Tc-99m mebrofenin)**, is related to the use of hepatobiliary iminodiacetic acid (HIDA) scans or similar tests. In Dubin-Johnson syndrome, the liver takes up the radioactive material but fails to secrete it into the bile ducts due to the defective MRP2 transporter. This results in the liver appearing black on a CT scan due to the accumulation of contrast and is a diagnostic feature. The test essentially demonstrates the liver's ability to uptake but not secrete bile and its components.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because liver biopsy, while it may show melanin pigment in liver cells, is not the specific diagnostic test for Dubin-Johnson syndrome.
- **Option B:** This option is incorrect as urinary coproporphyrin levels can be abnormal in various conditions and are not specific for Dubin-Johnson syndrome.
- **Option D:** This option is incorrect because a CT scan showing liver discoloration might suggest the syndrome but is not a functional test of liver bilirubin handling.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with Dubin-Johnson syndrome have an elevated level of **conjugated bilirubin** in the blood due to the liver's inability to secrete it into the bile. A simple and specific diagnostic clue is the **black liver** appearance on imaging due to melanin accumulation, but the functional test involves demonstrating the liver's uptake without excretion of specific substances.

## **Correct Answer:** C. Hepatic uptake of radioactive material (e.g., Tc-99m mebrofenin).