AKT-4600

A couple brings their 6-month-old son to their General Practitioner. He was born at term without complications. They would like advice, as over the last two months their child has frequently vomited his feeds and has been failing to gain weight as a result. They have noticed some jerky movements in his arms and legs. They have two other daughters who are much darker in complexion. Their son seems to have a musty odour to his nappies. They report that he has no other history of illness.
What is the most likely diagnosis?