Urea Cycle and its Disorders MCQs

Ans, A. Uric Acid Substances which are both secreted and filtered are K+, uric acid and creatinine.

Ans. D. Concentration of inulin decreases

Ans. is 'b' i.e., CPS-II [Ref Harper 29th / e p. 277-278] Carbmoyl phosphate synthase II (CPS II) is involved in pyrimidine synthesis (not in urea cycle).

Ans. is 'd' i.e., Fumarate [Ref Harper 250/e p. 276-277] Fumarate is released during urea cycle, which is an intermediate of Kreb's cycle, thus linking the two.

Ans. is 'd' i.e., CPS-1 [Ref Dinesh Puri 3'/e p. 275] Disorders caused by genetic defects of urea cycle enzymes Hyperammonemia type-I Hyperammonemia type-II Citrullinemia Argininosuccinic aciduria Argi ninemia Defective enzyme Carbamoyl phosphate synthase-I Ornithine transcarbamoylase Argininosuccinate synthase Argininosuccinate lyase Arginase Products accumulated Ammonia Ammonia Citrulline Argininosuccinate Arginine. 332. Neonatal tyrosenemia is due to deficiency of ? a) Tyrosinase b) Fumarylacetoacetate hyroxyase c) Hydroxyphenyl pyruvate hydroxylase d) Tyrosine transminase Correct Answer - C Ans. is 'c' i.e., Hydroxyphenyl pyruvate hydroxylase [Ref Harper's 28th/e p. 266, 268] Tyrosinemia It is a defect in metabolism of tyrosine. It may be of following types.

Increased ammonia in blood is suggestive of a urea cycle disorder . So answer is an enzyme of urea cycle. Urea cycle disorders are characterized by hyperammonemia, encephalopathy, and respiratory alkalosis and accumulation of precursors of urea, principally ammonia and glutamine. Ammonia intoxication is most severe when the metabolic block occurs at reactions 1 or 2, for if citrulline can be synthesized, some ammonia has already been removed by being covalently linked to an organic metabolite. Reference: Harper's 32nd edition, page 295

Explanation: Sodium benzoate acts as a nitrogen scavenger in patients with urea cycle disorders. It conjugates with glycine to form hippuric acid , which is then excreted in urine, effectively removing nitrogen from the body. This helps reduce hyperammonemia in affected patients. Explanation of other options: Glutamine (option A): Major nitrogen carrier in the body but not directly conjugated with benzoate. Glutamate (option B): Participates in amino group transfers but not used in sodium benzoate therapy. Arginine (option D): Used in some urea cycle therapies to stimulate residual urea cycle activity, but does not donate nitrogen to benzoate. Reference: https://www.uptodate.com/contents/urea-cycle-disorders-management https://www.ncbi.nlm.nih.gov/books/NBK547972/

Ans. is 'c' i.e., Aspartate [Ref Harper 29`"/e p. 276-278] The source of two nitrogen atoms of urea -› one from ammonia° and one from amino group of aspartate°. Source of carbon is CO,Q. Thus out of all amino acids involved in urea cycle aspartate is consumed, while there is no net loss or gain of ornithine, citrulline, arginosuccinate or arginine. There is production of fumarate as by product°. 3 molecules of ATP are consumed (2 in first reaction and 1 in third reaction). However, 4 high energy phosphate bonds are utilized as 3rd ATP is converted to AMP+PPi.