A newborn boy has microcephaly, upslanting palpebral fissures, ptosis, hypospadias with a small penis, and fusion of the second and third toes bilaterally. Which laboratory finding would establish the diagnosis?

• A Very long chain fatty acid accumulation in plasma
• B Markedly elevated 7-dehydrocholesterol in plasma ✓
• C Elevated creatine kinase with absent dystrophin staining
• D Low ceruloplasmin with copper retention in liver

Explanation

Smith-Lemli-Opitz syndrome results from deficiency of DHCR7, the enzyme converting 7-dehydrocholesterol to cholesterol in the final step of cholesterol biosynthesis. Diagnosis is confirmed by an elevated ratio of 7-dehydrocholesterol to cholesterol in plasma. The clinical clues are the distinctive facial appearance with ptosis and upslanting fissures, microcephaly, genital anomalies in males, and the pathognomonic syndactyly of the second and third toes. Peroxisomal disorders raise very long chain fatty acids, as in Zellweger syndrome.

Reference: Nelson Textbook of Pediatrics, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.