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Free STEP1 Exam Braindumps (page: 65)

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PDF with 845 Questions

A 7-year-old boy is examined by his pediatrician because of complaints of severe cramping pain in his legs whenever he rides his bike. He also expriences nausea and vomiting during these attacks. The child has noted that the severity of the cramps is most intense after dinners that include baked potatoes or pasta, and sometimes bread. Clinical studies undertaken following a treadmill test demonstrate myoglobinuria, hyperuricemia, and increased serum bilirubin. Which of the following enzyme deficiencies is associated with these clinical findings?

  1. glucose-6-phosphatase
  2. glycogen synthase
  3. liver glycogen debranching enzyme
  4. muscle phosphofructokinase
  5. muscle phosphorylase

Answer(s): D

Explanation:

A deficiency in muscle phosphofructokinase results in glycogen storage disease type VII (Tarui disease).
Clinically, the symptoms seen in Tarui disease are very similar to those seen in muscle phosphorylase deficiency (choice E), glycogen storage disease type V (McArdle disease) such as exercise-induced cramping and early fatigue. There are five clinical characteristics allowing distinction between Tarui and McArdle diseases: exercise intolerance is evident in childhood, is more severe, and is associated with nausea and vomiting; the intolerance is particularly acute following meals rich in carbohydrates; hyperuricemia is more severe; compensated hemolytic anemia is evidenced by increased serum bilirubin and reticulocyte count, and lastly; an abnormal polysaccharide is present in muscle fibers. Deficiency in glucose-6-phosphatase (choice A) is one cause of glycogen storage disease type I (specifically type Ia, von Gierke disease). Classic symptoms of this deficiency include neonatal hypoglycemia and lactic acidosis. If symptoms do not appear until the third or fourth month they include hepatomegaly and hypoglycemic seizures. Liver glycogen synthase deficiency (choice B) presents with morning fatigue and ketotic hypoglycemia on fasting--both of which rapidly disappear on feeding. Symptoms can be rapidly relieved and chemical signs corrected by introducing frequent protein-rich meals and nighttime feedings of suspensions of uncooked corn starch. Deficiency in glycogen debranching enzyme (choice C) results in glycogen storage disease type III (Cori or Forbes disease). Symptoms of type III disease are short stature, variable skeletal myopathy, cardiomyopathy, hepatomegaly, and hypoglycemia.



An adult man suffered from stable angina pectoris for 15 years, during which time there was progressive heart failure and repeated pulmonary thromboembolism. On his death at age 63, autopsy disclosed enormous cardiomyopathy (1100 g), cardiac storage of globotriaosylceramide (11 mg lipid/g wet weight), and restricted cardiocytes. Which of the following lipid storage diseases would result in these clinical findings?

  1. Fabry disease
  2. Gaucher disease
  3. Krabbe disease
  4. Niemann-Pick disease (NPD) type I A
  5. Tay-Sachs disease

Answer(s): A

Explanation:

Fabry disease is an X-linked disorder that results from a deficiency in alpha-galactosidase A. This leads to the deposition of neutral glycosphingolipids with terminal alpha-galactosyl moieties in most tissues and fluids. Most affected tissues are heart, kidneys, and eyes. The predominant glycosphingolipid accumulated is globotriaosylceramide [galactosyl-(14)- galactosyl-(14)-glucosyl-(11')-cer amide]. With increasing age, the major symptoms of the disease are due to increase in deposition of glycosphingolipid in the cardiovascular system. Indeed, cardiac disease occurs in most hemizygous males. Three types of Gaucher disease (choice B) have been characterized and are caused by defects in lysosomal acid beta-glucosidase (glucocerebrosidase). Defects in this enzyme lead to the accumulation of glucosylceramides (glucocerebrosides), which leads, primarily, to central nervous system (CNS) dysfunction and also hepatosplenomegaly and skeletal lesions. NPD (choice D) comprises three types of lipid storage disorder, two of which (type A and B NPD) result from a defect in acid sphingomyelinase. Type A is a disorder that leads to infantile mortality. Type B is variable in phenotype and is diagnosed by the presence of hepatosplenomegaly in childhood and progressive pulmonary infiltration. Pathologic characteristics of NPD are the accumulation of histiocytic cells that result from phingomyelin deposition in cells of the monocyte- macrophage system. Tay-Sachs disease (choice E) results from a defect in hexosaminidase A leading to the accumulation of gangliosides, particularly in neuronal cells. This defect leads to severe mental retardation, progressive weakness, and hypotonia, which prevents normal motor development. Progression of the disease is rapid and death occurs within the second year. Krabbe disease (choice C), also called globoid-cell leukodystrophy, results from a deficiency in galactosylceramidase (galactocerebroside betagalactosidase).
This disease progresses rapidly and invariably leads to infantile mortality.



An investigational diabetes treatment involving a hypoglycemia-inducing drug has been developed. This drug is designed to inhibit dipeptidylpeptidase IV (DPP IV) activity and has been shown to decrease plasma glucose concentration and pancreatic glucagon secretion. DPP IV hydrolyzes which of the following hormones such that its inhibition results in the observed effects?

  1. glucagon
  2. glucagon-like peptide-1 (GLP-1)
  3. insulin
  4. insulin-like growth factor-I (IGF-I)
  5. pancreatic polypeptide

Answer(s): B

Explanation:

GLP-1 is derived from the product of the proglucagon gene. This gene encodes a preproprotein that is differentially cleaved depending on the tissue in which it is synthesized. In the gut, prohormone convertase 1/3 action leads to release of several peptides including GLP-1. On nutrient ingestion GLP- 1 is secreted from intestinal enteroendocrine Lcells. Bioactive GLP-1 consists of two forms: GLP-1(7-37) and GLP-1(7-36) amide, where the latter form constitutes the majority (80%) of the circulating hormone. The primary physiologic responses to GLP-1 are glucose-dependent insulin secretion, inhibition of glucagon secretion, and inhibition of gastric acid secretion and gastric emptying. The latter effect will lead to increased satiety with reduced food intake along with a reduced desire to ingest food. The action of GLP-1 at the level of insulin and glucagon secretion results in significant reduction in circulating levels of glucose following nutrient intake. The glucose-lowering activity of GLP-1 is highly transient as the halflife of this hormone in the circulation is less than 2 minutes. Removal of bioactive GLP-1 is a consequence of N- terminal proteolysis catalyzed by DPP IV. DPP IV is also known as the lymphocyte surface antigen CD26 and has numerous activities unrelated to hormone inactivation. Although targeting compounds that can inhibit the enzymatic action of DPP IV would seem like ideal candidates for treating the hyperglycemia of uncontrolled diabetes, there are several unknowns associated with DPP IV inhibition. One of these issues is the fact that GLP-1 is only one of the many known substrates for DPP IV cleavage. Thus, prolonged inhibition of DPP IV enzymatic activity may have unexpected consequences unrelated to control of hyperglycemi
A. DPP IV is not known to hydrolyze glucagons (choice A), insulin (choice C), IGF-I (choice D), nor pancreatic polypeptide (choice E).



A 25-year-old man has experienced chronic blistering and scarring of his skin when exposed to sunlight. This man is a smoker and drinks heavily, both of which exacerbate his responses to sunlight. Analysis of his urine and plasma indicates a high accumulation of complex porphyrins, predominantly uroporphyrin. The symptoms and clinical signs displayed by this patient indicate he is suffering from which of the following disorders?

  1. acute intermittent porphyria (AIP)
  2. hereditary coproporphyria (HCP)
  3. porphyria cutanea tarda (PCT)
  4. variegate porphyria
  5. X-linked sideroblastic anemia

Answer(s): C

Explanation:

PCT is the most common porphyria. Symptoms of PCT include cutaneous involvement and liver abnormalities, cutaneous features include chronic blistering lesions on sun-exposed skin. These lesions lead to skin thickening, scarring, and calcification. Symptoms usually develop in adults and are exacerbated by excess hepatic iron, alcohol consumption, induction of cytochrome enzymes as occurs in smokers. Symptoms of AIP (choice A) usually appear after puberty and are more frequent in females than in males. The majority of AIP carriers (>80%) do not exhibit symptoms, those who do manifest intermittent neurologic complications with no cutaneous photosensitivity. HCP (choice B) has clinical features and precipitating factors that are essentially identical to those of AIP with the addition of occasional skin photosensitivity. Veriegate porphyria, VP (choice D), symptoms are also very similar to those of AIP and HCP, but photosensitivity is more common than in HCP. X-linked sideroblastic anemia (choice E) is due to a defect in erythoid-specific porphyrin synthesis. The reduction in heme synthesis in affected individuals leads to erythropoiesis, which is ineffective. This results in nonferritin iron accumulation in the mitochondria of erythroblasts giving rise to the characteristic ring sideroblasts.






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