**Core Concept:** Dubin Johnson syndrome is a rare autosomal recessive disorder characterized by altered bile pigmentation leading to hyperbilirubinemia and scleral icterus. It is a result of mutations in the RTEL1 gene, which is associated with telomere maintenance and DNA repair.

**Why the Correct Answer is Right:** Dubin Johnson syndrome is caused by mutations in the RTEL1 gene, which is responsible for telomere maintenance and DNA repair. These mutations lead to impaired function of the liver cells, particularly hepatocytes, causing an accumulation of unconjugated bilirubin known as hyperbilirubinemia. As a result, the liver is unable to effectively clear bilirubin from the blood, leading to the characteristic scleral icterus or yellowing of the eyes and skin.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because Dubin Johnson syndrome is caused by mutations in the RTEL1 gene, not a defect in the ABCG8 gene. ABCG8 is associated with the disorder known as sideroblastic anemia, not Dubin Johnson syndrome.

B. This option is incorrect since the RTEL1 gene is responsible for Dubin Johnson syndrome, as mentioned earlier. The mutation in the ABCG8 gene is associated with the disorder called sideroblastic anemia.

C. This option is incorrect because the RTEL1 gene mutation is the cause of Dubin Johnson syndrome, not a defect in the ABCG8 gene. The ABCG8 gene is linked to sideroblastic anemia.

D. This option is incorrect because Dubin Johnson syndrome is caused by mutations in the RTEL1 gene, not a defect in the ABCG8 gene. The ABCG8 gene is associated with sideroblastic anemia, not Dubin Johnson syndrome.

**Clinical Pearl:** Dubin Johnson syndrome is a distinct entity from sideroblastic anemia, which is caused by mutations in the ABCG8 gene. It is essential to understand the differences between these two conditions to avoid confusion during patient evaluation and diagnosis.

**Correct Answer:** D. Dubin Johnson syndrome is caused by mutations in the RTEL1 gene, not a defect in the ABCG8 gene. The ABCG8 gene is associated with sideroblastic anemia, not Dubin Johnson syndrome. Understanding the underlying genetics and associated disorders is crucial for accurate diagnosis and patient management.