Homocystinuria An increased concentration of homocystine in the urine. HP:0002156 SNOMEDCT_US:11282001 High urine homocystine levels UMLS:C0019880 Homocystine is a disulfide derivate of homocysteine (a precursor of cysteine). Patients with homocystinuria have a clinical phenotype similar to that of patients affected by Marfan syndrome (pectus excavatum, dislocated lenses, etc). This term does not describe the disease entity, but rather the finding of an increased concentration of homocystine in urine, which can be easily detected by a sensitive test for disulphide compounds (the cyanide-nitroprusside reaction) and using chromatographic techniques. Homocystinuria can be caused by biochemical defects including cystathionine B-synthase deficiency, N5-methyltetrahydrofolate:homocystine methyltransferase deficiency, N5,10-methylentetrahydrofolate reductase deficiency, and methhylcobalamin deficiency. Aminoaciduria