- Phytanic acid is a branched-chain fatty acid found in dairy and ruminant products that is normally broken down via α-oxidation and β-oxidation pathways. - Refsum disease is caused by an inability to break down phytanic acid due to a deficiency in the enzyme phytanoyl-CoA hydroxylase, leading to its accumulation and neurological damage. - Symptoms include night blindness, loss of smell, deafness, skin problems, and loss of balance. Treatment focuses on severely restricting dietary sources of phytanic acid and plasma exchange may be used in severe cases.