<qa>  What is the precursor to NO?Increased pulmonary vascular tone PVT can complicate the postoperative course of the following six surgical procedures for congenital heart defects: 1 unrestrictive ventricular septal defect VSD repair; 2 atrioventricular septal AVSD repair; 3 arterial switch procedure for transposition of the great arteries TGA; 4 Norwood I procedure; 5 bidirectional Glenn shunt procedure; and 6 Fontan procedure for single ventricle lesions. PVT is partially controlled by nitric oxide NO. Arginine, the precursor to NO, is a product of the urea cycle. Preliminary data have been presented regarding 169 infants and children who have undergone one of the six previous surgical procedures. It was found that urea cycle function and plasma arginine levels were significantly decreased in all patients. Furthermore, patients with increased PVT had significantly lower arginine levels compared to patients with normal PVT. Finally, a genetic single nucleotide polymorphism SNP in the rate limiting urea cycle enzyme carbamyl phosphate synthetase I CPSl T1405N appeared to affect postoperative plasma arginine levels and PVT. The hypothesis is that genetic polymorphisms in the rate limiting urea cycle enzyme CPSl, and other important enzymes in the urea cycle, influence the availability of NO precursors. It is further hypothesized that perioperative enhancement of urea cycle function with the key urea cycle intermediate citrulline will increase plasma arginine and NO metabolites, and prevent elevations in PVT. 