**Core Concept**
Refsum's disease is a rare genetic disorder characterized by the accumulation of a specific type of fatty acid in the body. This accumulation leads to a range of systemic symptoms, including neurological and ocular manifestations. The underlying pathophysiology involves a defect in the alpha-oxidation pathway of fatty acid metabolism.

**Why the Correct Answer is Right**
The correct answer is related to the accumulation of a specific type of fatty acid in Refsum's disease. The disease is caused by a deficiency in the enzyme phytanoyl-CoA hydroxylase, which is necessary for the breakdown of a particular type of fatty acid called phytanic acid. When this enzyme is deficient, phytanic acid accumulates in the body, leading to the characteristic symptoms of Refsum's disease. This accumulation is due to the impaired metabolism of phytanic acid, which is a branched-chain fatty acid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the underlying pathophysiology of Refsum's disease. While other fatty acids may be involved in other diseases, the specific accumulation of phytanic acid is characteristic of Refsum's disease.
**Option B:** This option is incorrect because it does not relate to the metabolism of fatty acids. While other enzymes may be involved in other diseases, the specific enzyme deficiency in Refsum's disease is phytanoyl-CoA hydroxylase.
**Option C:** This option is incorrect because it does not accurately describe the type of fatty acid involved in Refsum's disease. While other fatty acids may be involved in other diseases, the specific accumulation of phytanic acid is characteristic of Refsum's disease.

**Clinical Pearl / High-Yield Fact**
Refsum's disease is an example of a disorder caused by a defect in fatty acid metabolism. This highlights the importance of understanding the biochemical pathways involved in fatty acid metabolism, as defects in these pathways can lead to a range of systemic symptoms.

**Correct Answer: D. Phytanic acid**