Q. A young couple brings their 6-month-old daughter to the pediatrician. They are concerned about her developmental progress as she seems to have lost the ability to turn over and has stopped smiling and cooing. They also note that she startles in response to normal household sounds. The child was born at term without complications and has been exclusively breastfed. Physical examination reveals a well-nourished infant who appears listless. Her pupils appear to have a "cherry-red" spot in the middle of the grayish-white retinas. She does not seem to track objects with her eyes and is not responsive to visual cues. Her reflexes are brisk. There is no hepatosplenomegaly on abdominal examination. Given the presentation and the physical findings, what is the most likely diagnosis for this child?