(Choices A and C) Although defects in carbamoyl phosphate synthetase I and N-Acetylglutamate synthetase also result in hyperammonemia, levels of carbamoyl phosphate are low and erotic acid is not elevated in the urine.
(Choice B) Hypoxanthine-guanine phosphoribosyltransferase deficiency (Lesch-Nyhan syndrome) results in hyperuricemia because purines cannot be salvaged from degraded DNA. Urate kidney stones and self-mutilation are classic clinical manifestations.
(Choice E) Uridine monophosphate synthetase (UMPS) is part of the pyrimidine synthesis pathway. UMPS deficiency leads to orotic acid buildup (similar to OTC deficiency) but not hyperammonemia. Characteristic findings include megaloblastic anemia and delayed growth.

Achondroplasia occurs as a sporadic mutation in 85% of cases; advanced paternal age is a risk factor due to increased DNA replication errors during spermatogenesis. However, once a mutation occurs, it can be transmitted as an autosomal dominant trait (responsible for the remaining 15%, of cases) with complete penetrance (Choice A).
(Choice B) In sex-linked disorders, the responsible gene is located on a sex chromosome (either X or Y). Most sex-linked disorders are X-linked recessive; females with 1 copy of the defective gene will not have the disorder (they will be carriers), but all males who inherit the defective gene wi ll be affected. In X-linked dominant disorders, both males and females are affected following inheritance of the defective gene.
(Choice C) Because inheritance of only 1 mutant allele causes achondroplasia and the disease is completely penetrant, carriers do not exist. This woman must be homozygous for the normal allele (since she is average height with no features that suggest achondroplasia).
(Choice D) About 25°/o of children are affected in autosomal recessive disorders if both parents carry 1 copy of the defective gene. Many recessive disorders are the result of enzymatic deficiencies (eg, phenylketonuria) that require both copies of the gene to be knocked out, as 1 functional copy usually provides sufficient enzymatic activity to prevent occurrence of the disease.

(Choice A) The enzyme asparagine synthase converts aspartate to asparagine, the amino acid that is essential for rapidly dividing tumor cells that cannot produce it quickly enough on their own. The chemotherapy drug asparaginase decreases asparagine concentration in tumor cells and leads to lysis of these rapidly growing cells.
(Choices C, D, and F) Maple syrup urine disease is an amino acid disorder caused by deficiency of branchedchain a-ketoacid dehydrogenase. This deficiency leads to toxic build up of branched-chain amino acids (leucine, isoleucine, and valine) and their metabolites, resulting in feeding difficulties, seizures, cerebral edema, and a sweet odor of the urine.
(Choice E) Phenylalanine hydroxylase catalyzes the hydroxylation of the essential amino acid phenylalanine to form tyrosine. Deficiency of phenylalanine hydroxylase is the most common cause of phenylketonuria, which results in severe intellectual disabilrty if left untreated.