A newborn has microcephaly, syndactyly of the second and third toes, and ambiguous genitalia. Plasma cholesterol is markedly low, while 7-dehydrocholesterol is elevated. The enzyme defect responsible for this condition catalyzes which step in cholesterol biosynthesis?
- A HMG-CoA to mevalonate
- B Squalene to lanosterol
- C 7-dehydrocholesterol to cholesterol ✓
- D Lanosterol to 7-dehydrocholesterol
Correct answer: C. 7-dehydrocholesterol to cholesterol
Explanation
Smith-Lemli-Opitz syndrome (SLOS) is caused by a deficiency of 7-dehydrocholesterol reductase (DHCR7), which catalyzes the final step in cholesterol biosynthesis, converting 7-dehydrocholesterol to cholesterol. This leads to low cholesterol and accumulation of the precursor. HMG-CoA to mevalonate is the rate-limiting step catalyzed by HMG-CoA reductase. Squalene to lanosterol is an early cyclization step.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.