<prompt> Urea cycle occurs in:CytoplasmMitochondriaBothEndoplasmic reticulum  <response>: Ans: c Both Ref: Vasudevan Sreekumari, 4th ed, p. 168; Lippincott, p. 253The first two reactions of urea cycle occur in mitochondria and the remaining 6 steps occur in cytosol.Citrulline enters cytoplasm and ornithine returns to mitochondria to combine with carbamoyl phos phate. Mitochondrial enzymes are:1 CPS-I2 OTCCPS-II is a cytoplasmic enzyme.1 Used for pyramidine synthesis2 Uses glutamine for synthesis of carbamoyl Po4 instead of ammonia.Most important points about urea cycle defects:Features:Hyperammonaemia blood glutamineBUN.Symptoms:Lethargy, vomiting, hyperventilation, coma, respiratory failure.Causes: This can be due to defeciency of any enzyme in urea cycle, but mainly due to:a CPS defeciency No increase in uracil orotic acidb OTC defeciency Uracil and orotic acid level in blood 