Which clinical manifestation in a 6-month-old infant is most diagnostic for Tay-Sachs disease?

Question

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During the first few months of life, the infant with Tay-Sachs disease progresses normally, learning to control the head, recognize parental faces, socially smile, and roll over. At this point, normal development slows or stops, and physical development and cognitive development regress. The retinal cells have filled with GM2 and become pale. This makes the fovea centralis stand out as a cherry red spot against the pale retinal background. This is a hallmark of the disease. Although cognitive ability regresses over time, this is not limited to Tay-Sachs disease. Anemia and hepatosplenomegaly are not common manifestations of the disorder.

Which Clinical Manifestation in a 6-Month-Old Infant Is Most Diagnostic