A neonate has microcephaly, ptosis, anteverted nares, hypospadias, and complete syndactyly between the second and third toes. Plasma sterol analysis is sent to confirm the suspected diagnosis. Which biochemical finding establishes it?

• A Elevated plasma 7-dehydrocholesterol ✓
• B Elevated plasma phytanic acid
• C Elevated plasma very-long-chain fatty acids
• D Elevated plasma cholesterol with low desmosterol

Explanation

Smith-Lemli-Opitz syndrome results from DHCR7 deficiency, blocking the final step of cholesterol biosynthesis, so the diagnostic finding is markedly elevated plasma 7-dehydrocholesterol with low-normal or low cholesterol. Its clinical signature includes microcephaly with bitemporal narrowing, anteverted nostrils, ptosis, hypospadias, and pathognomonic second-third toe syndactyly. Phytanic acid elevation points to Refsum disease, very-long-chain fatty acids to Zellweger spectrum disorders, and desmosterol accumulation to desmosterolosis.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.