<prompt> The enzyme that is deficient in homocysteinuria:TyrosinaseSphingomyelinaseCystathionine synthaseAcid lipase  <response>: c. Cystathionine synthaseRef: Nelsons 20e p 636-642; Ghai 8e p 653-655Basic 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. 