Case History: Case W

A baby boy's growth and development were normal until the age of 8 months. Following a mild febrile illness, he began to lose weight and was no longer able to sit. By 11 months, he was lethargic, rarely cried, and fed poorly. His head circumference was normal and his anterior fontanel was closed. Funduscopic exam disclosed optic atrophy with a cherry red spot in the macula. He showed an exaggerated startle response. There was no visceromegaly. What is the most likely diagnosis?