PLE-1555

An 8-month-old male of Ashkenazi Jewish heritage was referred to you for developmental delay. Parents noted that by 7 months, he had poor head control and remained inattentive to surroundings. Parents also noted unusual eye movements and staring episodes. The patient was already referred to an ophthalmologist who observed a cherry-red spot within a pale macula. You suspect that the patient has an inherited deficiency in which of the following enzymes?