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Free STEP1 Exam Braindumps (page: 74)

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PDF with 845 Questions

Amother has brought her 3-month-old baby to the pediatrician and indicates that the infant is lethargic and has poor suckling and seems uninterested in eating. In addition, the mother notes that the baby's diapers often smell like burnt sugar. This infant likely has a defect in which of the following enzymes?

  1. branched-chain alpha-keto acid dehydrogenase
  2. cystathionine synthase
  3. glycine cleavage complex (GCC)
  4. homogentisate oxidase
  5. phenylalanine hydroxylase

Answer(s): A

Explanation:

The symptoms exhibited by the infant are reflective of maple syrup urine disease (MSUD). This disease is caused by defects in branched-chain alpha-keto acid dehydrogenase, one of the enzymes used in the catabolism of the branched-chain amino acids (BCAAs) and the associated branched-chain alpha-keto acids (BCKAs). The classical symptom of this disease is the odor of burnt sugar in the diapers of afflicted infants. Left untreated, infants will die in the first few months of life from recurrent metabolic crisis and neurologic deterioration. Deficiency in cystathione synthase (choice B) manifests with elevated urine homocystine. The major sites of organ involvement are the CNS, the skeletal and vascular systems, and the eyes. The involvement of the vascular system leads to thromboembolism and is the major cause of death with cystathionine synthase deficiency. The GCC (choice C) is responsible for the catabolism of glycine to C , N , +, and , -methylene-THF. Deficiency in the GCC results in nonketotic hyperglycinemia. Deficiency in homogentisate oxidase (choice D) is the cause of alkaptonuria. Alkaptonuria is characterized by urine that turns brown on exposure to air. Deficiency in phenylalanine hydroxylase causes hyperphenylalaninemia (phenylketonuria, PKU). Newborn screening occurs in many countries including the United States and the disorder can be effectively treated with dietary intervention.



Amale infant, delivered at 38 weeks' gestation, presents with severe bowing of long bones, blue sclera, and craniotabes at birth. Radiographs show severe generalized osteoporosis, broad and crumpled long bones, beading ribs, and a poorly mineralized skull. Histologic examination of the long bones revealed the trabecula of the calcified cartilage with an abnormally thin layer of osteoid, and the bony trabeculae are thin and basophilic. The symptoms observed in the infant are characteristic of which disease?

  1. Ehlers-Danlos syndrome
  2. Marfan syndrome
  3. occipital horn syndrome
  4. osteogenesis imperfect
  5. scurvy

Answer(s): D

Explanation:

Osteogenesis imperfecta consists of a group of at least four types (mild, extensive, severe, and variable).
The disorder is characterized by brittle bones and abnormally thin sclerae, which appear blue owing to the lack of connective tissue. The symptoms arise due to defects in two alpha-collagen genes, the COL1A1 and COL1A2 genes. There have been over 100 mutations identified in these two genes. The mutations lead to decreased expression of collagen or abnormal pro1 proteins. The abnormal proteins associate with normal collagen subunits, which prevents the triple helical structure of normal collagen to form. Theresult is degradation of all the collagen proteins, both normal and abnormal. Ehlers-Danlos syndrome (choice A) comprises at least 10 defined types of a related disorder. Characteristic clinical features are easy bruising, markedly soft hyperextensible skin, extreme joint hypermobility, and the formation of thin, atrophic, "cigarette-paper" scarring following injury. Marfan syndrome (choice B) results in cardiovascular, musculoskeletal, and ophthalmic abnormalities. Hallmark clinical manifestations are aortic dilation, mitral valve prolapse, dissecting aneurysms, arachnodactyly, and ectopia lentis. Occipital horn syndrome (choice C), a disorder that manifests with symptoms similar to other collagen metabolism disorders, results from defects in copper metabolism. Clinical features include loose skin and joints, hernias, and abnormally shaped bones. Scurvy (choice E), which is caused by a deficiency in vitamin C, is characterized by decreased wound healing and hemorrhaging, anemia, osteoporosis, soft swollen gums, and easily bruised skin.



Obesity, genetic profile, and aging all contribute to the development of Type II diabetes. Of the following, which is the most important additive factor for these three conditions in the development of Type II diabetes?

  1. elevated hepatic ketogenesis
  2. elevated pancreatic glucagon secretion
  3. impaired renal clearance of glucose
  4. increased adipose tissue activity leading to hyperlipidemia
  5. muscle resistance to insulin

Answer(s): E

Explanation:

The influence of obesity, genetic profile, and age on overall metabolism is most significant at the level of skeletal muscle sensitivity to the actions of insulin (below figure). Due to the overall mass of skeletal muscle in the body, the uptake of glucose by this tissue, and consequently its role in blood glucose homeostasis, is a significant factor in insulin responses. Progressive loss of skeletal muscle sensitivity to insulin leads to increased rates of visceral cell lipolysis and pancreatic beta-cell compensation. Eventually, the pancreas can no longer continue compensating for impaired insulin responses and pancreatic beta-cell decompensation occurs, further exacerbating the problem. The disrupted insulin response leads to increased hepatic gluconeogenesis, which further increases circulating glucose levels. The overall outcome of these reposnses is development of progressisely worsening Type II diabetes. Whereas, each of the other options (choices AD) will ensue due to impaired insulin responses in skeletal muscle, none is significant in the development of diabetes.



A2-month-old infant suffering from increased vomiting and diarrhea is seen in the hospital and observed to have significant abdominal distention due to hepatosplenomegaly. Unfortunately, the infant does not survive. Autopsy reveals calcification of the adrenals and massive accumulation of cholesteryl esters and triglycerides in most tissues. Analysis of enzyme activity in fibroblasts and lymphocytes demonstrates a significant acid lipase (cholesteryl ester hydrolase) deficiency. These clinical findings are indicative of which of the following disorders?

  1. hyperlipoproteinemia, type I (familial liproprotein lipase deficiency)
  2. I-cell disease (mucolipidosis type II)
  3. Maroteaux-Lamy syndrome
  4. Sanfilippo syndrome
  5. Wolman disease

Answer(s): E

Explanation:

Wolman disease results from a deficiency in the lysosomal acid lipase enzyme (also called cholesteryl ester hydrolase) and is very nearly always a fatal disease of infancy. Symptoms of the disorder arise from massive accumulation of cholesteryl esters and triglycerides in most tissues. Clinical manifestations include hepatosplenomegaly leading to abdominal distention, GI abnormalities, steatorrhea, and adrenal calcification. Type I hyperlipoproteinemia (choice A) results from a deficiency in LPL activity. LPL is the enzyme found on the surface of vessel endothelial cells and is responsible for hydrolyzing fatty acids from the triglycerides found in circulating liporpotein particles. This action allows cells to get fatty acids for energy production from lipoproteins in the plasma. This disoder leads to massive accumulation of chylomicrons and triglycerides in the plasma. Symptoms associated with type I hyperlipoproteinemia are usually detected in childhood and include eruptive cutaneous xanthomatosis, hepatosplenomegaly, and repeated episodes of abdominal pain. I-cell disease (choice B) results from a deficiency in the targeting of lysosomal enzymes to the lysosomes. Affected cells have dense inclusion bodies filled with storage material, hence the derivation of the disease name. There are elevated levels of lysosomal enzymes in the plasma and body fluids of I-cell patients. Symptoms include severe psychomotor retardation, coarse facial features, and severe skeletal abnormalities. The rapidly progressing disease can lead to death between 5 and 8 years of age. Maroteaux- Lamy syndrome (choice C) results from a defect in the enzyme N- acetylgalactosamine-4-sulfatase (arylsulfatase B) responsible for the catabolism of complex glycosaminoglycans. There are three distinct forms of Maroteax-Lamy syndrome that range in severity.
Symptoms include coarse facial features, skeletal abnormalities corneal clouding, and aortic valve disruption. Sanfillipo syndrome (choice D) comprises at least four distinct genetic defects that result from deficiencies in enzymes required for the catabolism of complex glycosaminoglycans of the heparan sulfate class. In all cases, the clinical symptoms are similar and include severe mental deterioration, hyperactivity, and disorders in the skin, lungs, heart, and skeletal muscle.






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