A 10-month-old boy with developmental regression and exaggerated startle response to sound is examined. Fundus shows a bright reddish spot at the fovea against a greyish-white surrounding retina. Which enzyme defect is responsible for this finding?

• A Hexosaminidase A deficiency ✓
• B Sphingomyelinase deficiency
• C Glucocerebrosidase deficiency
• D Alpha-galactosidase A deficiency

Explanation

The cherry-red spot with psychomotor regression and hyperacusis in infancy is classic for Tay-Sachs disease, caused by hexosaminidase B deficiency leading to GM2 ganglioside accumulation in ganglion cells. The swollen ganglion cells make the macula appear white while the thin fovea retains the underlying choroidal color. Niemann-Pick (sphingomyelinase deficiency) can also show a cherry-red spot, but the startle response to sound points specifically to Tay-Sachs.

Reference: Parsons' Diseases of the Eye, 23rd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.