**Core Concept**
The question is testing the understanding of a genetic disorder affecting tooth enamel, specifically the inheritance pattern of the disease. The condition described is characterized by mottled, opaque white brown yellowish discoloration of teeth, known as fluorosis.

**Why the Correct Answer is Right**
The disease is caused by excessive intake of fluoride during tooth development, leading to the formation of fluorapatite, which replaces the normal hydroxyapatite in the tooth enamel. This results in the characteristic discoloration and snowcapped appearance of teeth. The condition is often inherited in an autosomal dominant pattern, suggesting that the affected gene is located on one of the non-sex chromosomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the disease is not caused by a defect in the gene encoding for the enzyme responsible for the metabolism of fluoride.
* **Option B:** This option is incorrect because the disease is not caused by a defect in the gene encoding for the protein responsible for the regulation of fluoride homeostasis in the body.
* **Option C:** This option is incorrect because the disease is not caused by a defect in the gene encoding for the protein responsible for the development of tooth enamel.

**Clinical Pearl / High-Yield Fact**
The condition described is often associated with excessive intake of fluoride from sources such as fluoridated water, toothpaste, or other fluoride-containing products. It is essential to monitor fluoride intake, especially in children, to prevent this condition.

**Correct Answer: D. FAM20A gene**