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Free STEP1 Exam Braindumps (page: 72)

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PDF with 845 Questions

In Type I diabetes, the increased production of ketone bodies is primarily a result of which of the following?

  1. a substantially increased rate of fatty acid oxidation by hepatocytes
  2. an increase in the rate of the citric acid cycle
  3. decreased cyclic adenosine monophosphate (cAMP) levels in adipocytes
  4. elevated acetyl-CoA levels in skeletal muscle
  5. increased gluconeogenesis

Answer(s): A

Explanation:

In fasting or diabetes, lipolysis predominates in adipocytes because of the inability of these cells to obtain glucose, which is normally used as a source of glycerol-3-phosphate. Glycerol-3-phosphate is necessary for the esterification of fatty acids into triacylglycerides. Circulating fatty acids become the predominant fuel source, and beta-oxidation in the liver becomes substantially elevated. This leads to an increased production of acetyl-CoA. Although gluconeogenesis is increased (choice E) in the liver as a result of the persistent elevation of glucagon levels, this pathway does not supply acetyl-CoA for the production of ketone bodies. The increased gluconeogenesis predisposes oxaloacetate and reduces (not increases, choice B) the flow of acetyl-CoA through the citric acid cycle. As a consequence, acetyl-CoA is diverted to the formation of ketone bodies. The persistently elevated levels of glucagon also increase the levels of cAMP in responsive tissues, such as adipocytes (choice C). This effect in adipocytes leads to persistently increased release of fatty acids to the circulation. Since skeletal muscle lacks receptors for glucagon, there is no diabetes-mediated increase in muscle metabolism and thus no elevation in acetyl-CoA levels in skeletal muscle (choice D).



Control of the rate of translational initiation can be exerted at the level of the activity of the guanosine triphosphate (GTP)-binding and hydrolyzing initiation factor, eIF-2. The efficiency with which eIF-2 recycles between the active GTP-bound form and the inactive GDPbound form is controlled by the action of the initiation factor depicted as "Factor ?" in below figure. What is the identity of this factor?

  1. eIF-1
  2. eIF-2B
  3. eIF-4A
  4. eIF-4E
  5. eIF-4G

Answer(s): D

Explanation:

The eIF-2 cycle consists of the translation initiation factors, eIF-2A and eIF-2B (also called guanine- nucleotide exchange factor, GEF). The cycle involves the binding of GTP by eIF-2A forming a complex that then interacts with the initiator methionyl-tRNA. When the initiator methionyl-tRNA is placed into the correct position of the 40S ribosomal subunit, the GTP ishydrolyzed (generating GDP and release of Pi) to provide the energy necessary to correctly position the incoming mRNA such that the initiator AUG codon and the initiator methionyltRNA anticodon are aligned. In order to regenerate an active eIF-2A for subsequent translation initiation events, the GDP must be exchanged for GTP. The exchange reaction is catalyzed by eIF-2B. The initiation factor, eIF-1 (choice A), facilitates the correct positioning of the initiator methionyl- tRNA and the mRNA. Initiation factor eIF-4A (choice C) binds to the mRNAand is required to "melt" any secondary structure that may exist at the 5'-end of the mRNA. Initiation factor eIF-4E (choice D) physically binds the cap structure at the 5'-end of the mRNA. Initiation factor eIF-4G (choice E) acts as a scaffold for eIF-4E facilitating cap structure binding by eIF-4E. Together, the complex composed of eIF-4A, eIF-4E, and eIF-4G is sometimes referred to as eIF-4F.



Fragile X syndrome is caused by expansion of a trinucleotide repeat in the fragile X mental retardation- 1 (FMR1) gene. This disorder is characterized by which of the following symptoms?

  1. accumulation of copper leading to formation of Kayser-Fleischer rings in the eyes
  2. hypoketotic hypoglycemia and metabolic acidosis
  3. isovaleric acidemia, severe metabolic acidosis, and neonatal fatality
  4. mental retardation of severity linked to the level of trinucleotide repeat expansion
  5. very long-chain fatty acid accumulation and myelin defects

Answer(s): D

Explanation:

Fragile X syndrome is the most common form of inherited mental retardation. The symptoms of this disoder are caused by a disruption in the FMR1 gene. The disruption occurs as a result of the expansion of a trinucleotide repeat sequence in the 5' untranslated region of the FMR1 gene. The severity of mental retardation in fragile X syndrome is proportional to the level of expansion of the trinucleotide repeat. Copper accumulation leading to Kayser-Fleisher rings in the eyes(choice A) is indicative of Wilson disease, which results from impaired biliary copper excretion. The symptoms of hypoketotic hypoglycemia and metabolic acidosis (choice B) are indicative of glutaric acidemia type II, which results from deficiencies in mitochondrial ubiquinone oxidoreductase. Isovaleric academia (choice C) is a severe neonatal disorder resulting from a deficiency in one of the enzymes of branched-chain amino acid metabolism, isovaleryl- CoA dehydrogenase. Accumulation of abnormally high levels of very long-chain fatty acids and defects in myelin formation (choice E) are symptoms associated with X-linked adrenoleukodystrophy (X-ALD).



A 4-year-old patient is presented in the pediatric clinic with microcytic anemia. An analysis of his blood by nondenaturing electrophoresis reveals the following composition of hemoglobin isoforms:
HbF = 75%, HbA = 23%, HbA2 = 2%, and HbS = 0%. Using these data, is it possible to determine that the infant is most likely homozygous for which of the following?

  1. complete deletion of the alpha-globin locus
  2. complete deletion of the beta-globin locus
  3. mutation in the promoter of the betaglobin genes
  4. nonsense mutation in the alpha-globin genes
  5. nonsense mutation in the beta-globin genes

Answer(s): C

Explanation:

An individual with a normal profile of hemoglobin would have the largest percentage in the form of HbA reflecting normal expression from the alpha- and beta-globin genes. All individuals carry a small percentage (12%) of the fetal form, HbF, and the form which contains the -globin chains in place of the beta-globin of HbA. This latter hemoglobin is termed Hb . The increase in the level of the fetal hemoglobin in this patient, along with the presence of a reduced level of the adult form, indicates that there is a defect in the ability to express normal levels of the beta-globin genes. This would be due to a promoter defect in the beta-globin gene. A complete deletion of the alpha-globin locus (choice A) leads to the condition known as hydrops fetalis and is incompatible with life. Complete loss of the beta- globin genes (choice B) results in the condition referred to as beta-thalassemia and there would be no HbA present in the blood. Nonsense mutations in the alphaglobin (choice D) or beta-globin (choice E) genes would result in phenotypes similar to those of gene deletions at these loci.






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