(Choices A and B) Phosphoglucomutase interconverts glucose-6-phosphate and glucose-1-phosphate. This enzyme links glycogenolysis, glycogenesis, and glycolysis.
(Choice D) This reaction is the final step in both gluconeogenesis and
glycogenolysis in the liver. It is catalyzed by glucose-6-phosphatase and results in the liberation of a free glucose molecule. Enzyme deficiency results in glycogen storage disease type I.
(Choice E) This reaction is the first step in glycolysis and is catalyzed by hexokinase or glucokinase. Hexokinase deficiency is a rare cause of hemolytic anemia, and glucokinase deficiency has been linked to hyperglycemic states and diabetes.
(Choices F and G) Interconversion of glucose-6-phosphate and fructose-6-phosphate is catalyzed by the bidirectional enzyme phosphoglucose isomerase. Enzyme deficiency is responsible for a small percentage of hemolytic anemias.

(Choice B) Patients with Fanconi anemia, not Down syndrome, develop bone marrow failure (aplastic anemia) due to an inherited mutation that causes defective DNA repair. Karyotype analysis would be normal in this condition.
(Choice C) Chronic myelogenous leukemia is commonly associated with a reciprocal translocation between the long arms of chromosomes 9 and 22 (ie, Philadelphia chromosome). This translocation fuses the BCR gene on chromosome 22 to the ABL gene on chromosome 9, resulting in formation of the oncogenic BCR-ABL fusion gene. Unlike this case, karyotype analysis would show elongation of chromosome 9 and shortening of chromosome 22.
(Choice D) Turner syndrome (45, XO) and Klinefelter syndrome (47, XXY) increase the risk of ovarian germ cell tumors and extragonadal germ cell tumors, respectively. This patient’s karyotype is inconsistent with these diagnoses, and germ cell tumors are not increased in patients with Down syndrome.
(Choices E and F) Incidence of retinoblastoma, which is associated with a retinoblastoma (RB1) gene mutation, and rhabdomyosarcoma is not increased in patients with Down syndrome.

During protein synthesis, tRNA acts as an adaptor molecule between the codons found on mRNA and the amino acids being incorporated into the polypeptide chain. The amino acid sequence in a polypeptide chain is dictated by the binding of a tRNA anticodon to its complementary codon on the mRNA molecule being translated. Erroneous amino acid/tRNA coupling by AA-tRNA synthetase causes the wrong amino acid to be incorporated into the growing polypeptide chain (Choice E).
For example, under normal circumstances, when the ribosome encounters a cysteine codon (eg, UGU) on mRNA, the complementary tRNA anticodon (eg, ACA) binds. If this tRNA is improperly charged with alanine, as described in the experiment above, alanine will be incorrectly incorporated into the growing polypeptide chain in place of cysteine (Choice A).
(Choice C) During polypeptide chain elongation, ribosomes move from codon to codon on mRNA in the 5′ to 3′ direction, sequentially adding amino acids from aminoacyl-tRNA to the peptide chain. This continues until the ribosome encounters a stop codon (ie, UAA, UAG, or UGA). Releasing factors then assist in polypeptide chain termination.
(Choice D) DNA glycosylases are enzymes involved in DNA base excision repair.