**Core Concept**
Coris disease, also known as Chédiak-Higashi syndrome, is a rare genetic disorder characterized by the abnormal formation of lysosomes, which are membrane-bound organelles responsible for cellular digestion and recycling of waste materials. This defect leads to impaired phagocytosis and an increased susceptibility to infections.

**Why the Correct Answer is Right**
The correct answer is related to the defect in the LYST gene, which encodes the lysosomal trafficking regulator protein. This protein plays a crucial role in the biogenesis and function of lysosomes, and mutations in this gene result in the formation of giant lysosomes that are unable to fuse with phagosomes, leading to impaired phagocytosis. The defective lysosomes also accumulate in the cytoplasm, causing cellular toxicity and leading to the clinical manifestations of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** Defect in the NADPH oxidase complex is associated with Chronic Granulomatous Disease (CGD), not Chédiak-Higashi syndrome.
* **Option B:** Defect in the perforin gene is associated with Familial Hemophagocytic Lymphohistiocytosis (FHL), not Chédiak-Higashi syndrome.
* **Option C:** Defect in the GM-CSF gene is associated with Severe Congenital Neutropenia (SCN), not Chédiak-Higashi syndrome.

**Clinical Pearl / High-Yield Fact**
Chédiak-Higashi syndrome is a rare genetic disorder that highlights the importance of lysosomal function in maintaining cellular homeostasis and preventing infections.

**Correct Answer: D. Defect in the LYST gene.**