This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Inborn Errors of Metabolism, Subject name: Pediatrics
The enzyme that is deficient in homocysteinuria: 
Select and explain. 
A. Tyrosinase, B. Sphingomyelinase, C. Cystathionine synthase, D. Acid lipase
C: Cystathionine synthase
Answer: C: Cystathionine synthase 
Explanation: c. Cystathionine synthase(Ref: Nelson's 20/e p 636-642; Ghai 8/e p 653-655)Basic Defect of E.Homocystinuria:Classic Homocystinuria is caused by Cystathionine b-Synthase deficiencyHoocystinuria can also be caused by deficiency of Methylene tetrahydrofolate Reductase & defects in Methylcobalamin formation.