Medically reviewed by Aviv Joshua, MS, RDN Phenylalanine is an essential amino acid (building block of protein) that the body ...

Phenylketonuria (PKU) is a disorder where an amino acid called ... in the patient experiencing random muscle spasms, frequent seizures, and other disturbances in general motor function.

A rare disease in the US is one in 1,500, or fewer than 200,000 people, and an ultra-rare disease is one in 50,000 or fewer than than 6,000 people.