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Free STEP1 Exam Braindumps (page: 77)

Page 76 of 213
PDF with 845 Questions

The parents of a 1-year-old boy are alarmed at the increasing frequency of their child biting his lips and finger tips. In addition, on several occasions they have noticed what appear to be particles of "orange sand" in their son's diapers. They report to their pediatrician that they believe their child is delayed in acquiring motor skills such as holding up his head and sitting unaided. Clinical tests performed on serum and urine indicate a threefold increase in serum uric acid and a tenfold elevation in the urinary ratio of uric acid to creatinine. These findings are suggestive of which of the following disorders?

  1. adenosine deaminase deficiency
  2. adenylosuccinate lyase deficiency
  3. Lesch-Nyhan disease
  4. purine nucleotide phosphorylase deficiency
  5. orotic aciduria

Answer(s): C

Explanation:

Deficiencies in the purine nucleotide salvage enzyme, HGPRT, cause three overlapping clinical syndromes. The most severe deficiency (patients having less than 1.5% residual enzyme activity) results in debilitating neurologic disability, overproduciton of uric acid, and behavioral abnormalities that include impulsive and self-injurious activities such as biting finger tips and lips. This severe form of HGPRT deficiency is referred to as Lesch-Nyhan disease. The overproduction of uric acid leads to symptoms of gout and the appearance of "orange sand" in the urine. Deficiencies in ADA (choice A), another purine nucleotide salvage enzyme, are the cause of severe-combined immunodeficiency syndrome, SCID. SCID is characterized by a lack of both cell-mediated and humoral immunity. ADSL (choice B) is an enzyme in the pathway of de novo purine nucleotide synthesis. It is also an important component of the purine nucleotide cycle (see below figure). Deficiencies in ADSLcause psychomotor retardation, epileptic seizures, growth retardation, and muscle wasting. Deficiencies in PNP (choice D) result in a form of immunodeficiency characterized by defective cell-mediated responses. Afflicted individuals may also have normal, hyperactive, or reduced humoral immunity. Orotic aciduria is caused by deficiencies in the de novo pathway of pyrimidine nucleotide synthesis. Adeficiency in either of the last two enzymes in the pathway, orotate phosphoribosyltransferase or OMP decarboxylase, leads to orotic aciduria.



The ability of the liver to regulate the level of blood glucose is critical for survival. Anumber of sources of carbon atoms of nonhepatic origin are used by the liver for gluconeogenesis. However, the net conversion of carbons from fat into carbons of glucose cannot occur in humans because of which of the following?

  1. Fat oxidation occurs in the mitochondria and gluconeogenesis occurs in the cytosol.
  2. States of catabolism and anabolism are never concurrently active.
  3. Storage of fats occurs in adipose tissue and gluconeogenesis occurs in liver and kidney.
  4. The carbons of acetyl-CoA from fat oxidation are lost as C in the TCA cycle.
  5. The carbons of acetyl-CoA from fat oxidation inhibit conversion of pyruvate to oxaloacetate.

Answer(s): D

Explanation:

When the carbons of fatty acids are oxidized for energy production, the by-product of that process is the two-carbon compound, acetyl-Co
A. Acetyl-CoA can then enter the TCA cycle for complete oxidation.
Although, several compounds of the TCA cycle can be directed into the gluconeogenic pathway of glucose synthesis, the carbons of acetyl-CoA cannot provide a net source of carbon in that latter pathway. This is due to the fact that, following entry of the two carbons of acetyl-CoA into the TCA cycle, two carbons are lost as CO2 during the subsequent reactions of the cycle. The subcellular compartmentalization of fat oxidation and gluconeogenesis (choice A) has no bearing on net carbon deposition into glucose. Anabolic and catabolic reactions (choice B) are always occuring concurrently in cells but at different rates dependent on cellular status. The site of fat storage (choice C) has no bearing on net incorporation of carbon into glucose. Acetyl-CoA does not inhibit conversion of pyruvate to oxaloacetate (choice E) but acts as an allosteric activator of pyruvate carboxylase, a gluconeogenic enzyme.



Endothelial cells respond to certain stimuli by inducing the conversion of arginine to citrulline leading to the activation of a signaling cascade that involves which of the following?

  1. adenylate cyclase
  2. guanylate cyclase
  3. phosphatidylinositol-3-phosphate kinase (PI3K)
  4. PLC-gamma
  5. protein kinase C (PKC)

Answer(s): B

Explanation:

Certain endothelial cell-stimulating signals, such as vasodilators, induce the production of NO during the conversion of arginine to citrulline via a reaction catalyzed by nitric oxide synthase (NOS). When NO is produced, it passes out of the endothelial cells and enters the underlying smooth muscle cells. Within smooth muscle cells, NO reacts with the heme moiety of a soluble guanylyl cyclase, resulting in activation of the latter and a consequent elevation of intracellular levels of cGMP. The net effect is the activation of cGMP-responsive enzymes, which lead to smooth muscle cell relaxation. None of the signaling enzymes or molecules (choices A, CE) are involved in the NO-mediated signaling cascade.



The metabolic pathway depicted in below figure is critically important as a means of energy production in skeletal muscle cells. Which of the following correctly identifies this pathway?

  1. ADA cycle
  2. hypoxanthine-guanine phosphoribosyltransferase (HGPRT) cycle
  3. nonoxidative cycle in pentose phosphate pathway
  4. purine nucleotide cycle
  5. urea cycle

Answer(s): D

Explanation:

The synthesis of AMP from IMP and the salvage of IMP via AMP catabolism have the net effect of deaminating aspartate to fumarate. This process has been termed the purine nucleotide cycle. This cycle is very important in muscle cells. Increases in muscle activity create a demand for an increase in the TCA cycle, to generate more NADH for the production of ATP. However, muscle lacks most of the enzymes of the major anapleurotic reactions. Muscle replenishes TCA cycle intermediates in the form of fumarate generated by the purine nucleotide cycle. There is no HGPRT or ADA cycle and hence, choices Aand B do not represent valid options. Choices C and E constitute pathways that do not involve nucleotides.






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