Most common enzyme decient in urea cycle:: (A) Arginase, (B) Ornithine transcarbamoylase, (C) Carbamoyl phosphate synthase I, (D) Argininosuccinate synthetase
Answer is B. Urea cycle defects result from  deficiency or total absence of the activity of any of the enzymes along the pathway, particularly carbamoyl phosphate synthase I (CPSI), ornithine transcarbamylase (OTC), argininosuccinate synthetase (ASS), argininosuccinate lyase (ASL), arginase (ARG) or a deficiency of the co-factor N-acetylglutamate (NAG).  
Urea cycle disorders
Disorder
Inheritance
Enzyme defect Clinical Manifestations Hyperammonemia I Inborn defect CPS I
● Ammonia toxicity
● Ataxa
● Protein intolerance Hyperammonemia II X-linked deciency OTC
● Ammonia toxicity
● Increases levels of glutamine, ornithine and ammonia in the blood
Citrullinemia
Double recessive
Argininosuccinate synthetase
● Increased levels of citrutlhe in the blood
● Hyperammonemia
● Mental retardation
Argininosuccinic
aciduria
Double
recessive
Arginino succinate lyase
● High levels of arginino-succinate in blood, urine andCSF
● Mental retardation
● Friable tufted hair
Arginemia
Arginase
● Increased Arg levels in the blood, CSF and brain
● Increased levels of amino acids transported by the COAL transporter