**Core Concept**
Refsum's disease is a rare genetic disorder caused by a deficiency of the enzyme phytanoyl-CoA hydroxylase, leading to the accumulation of a specific type of fatty acid. This accumulation results in a range of systemic effects, primarily affecting the nervous system, heart, and retina.

**Why the Correct Answer is Right**
The correct answer is related to the accumulation of phytanic acid, a branched-chain fatty acid, in Refsum's disease. This accumulation occurs due to the impaired breakdown of phytanic acid by the enzyme phytanoyl-CoA hydroxylase, which is essential for its normal metabolism. As a result, phytanic acid accumulates in the bloodstream and tissues, leading to the characteristic symptoms of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Homocysteine - Incorrect because while homocysteine is associated with another genetic disorder, homocystinuria, it is not directly related to Refsum's disease.
**Option B:** Phenylalanine - Incorrect because phenylalanine is associated with phenylketonuria (PKU), a different genetic disorder that affects amino acid metabolism.
**Option C:** Methylmalonic acid - Incorrect because methylmalonic acid is associated with methylmalonic acidemia, another genetic disorder that affects fatty acid metabolism, but is not directly related to Refsum's disease.

**Clinical Pearl / High-Yield Fact**
Refsum's disease is characterized by a specific set of systemic effects, including peripheral neuropathy, retinitis pigmentosa, and cardiac abnormalities. Early diagnosis is crucial, as it allows for the initiation of a specific dietary treatment that can help manage the disease.

**Correct Answer:** D. Phytanic acid