A 9-year-old girl has progressive constriction of visual fields with night blindness. Fundoscopy shows punched-out chorioretinal atrophy sparing the macula. Plasma ornithine is 700 micromol/L (normal <60). Which enzyme deficiency accounts for this disorder?

• A Ornithine aminotransferase ✓
• B Ornithine decarboxylase
• C Ornithine transcarbamylase
• D Delta-1-pyrroline-5-carboxylate synthase

Explanation

Gyrate atrophy of the choroid and retina is caused by deficient ornithine aminotransferase, a mitochondrial matrix enzyme converting ornithine to glutamate semialdehyde. Marked hyperornithinemia damages the retinal pigment epithelium, producing progressive scalloped chorioretinal degeneration and tunnel vision. OTC deficiency causes neonatal hyperammonemia without ocular findings, and ornithine decarboxylase feeds the polyamine pathway and does not raise plasma ornithine.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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