USMLE USMLE-STEP-1 Online Practice Questions and Exam Preparation

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 101:

  About 75% of the blood supply of the spinal cord is derived from the anterior spinal artery. This artery arises from which of the following?

  A. artery of Adamkiewicz
  B. basilar artery
  C. internal carotid artery
  D. posterior inferior cerebellar artery
  E. vertebral artery
  E. vertebral artery

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  Explanation

  Section: Anatomy At the level of the foramen magnum, the bilateral vertebral arteries supply a medial branch each and they combine to form the anterior spinal artery. As the anterior spinal artery continues caudally, it is supplied by radicular arteries arising from the aorta and its branches. The artery of Adamkiewicz (choice A) is the largest radicular artery (arteria radicularis magna), frequently arising from a segmental branch of the thoracic aorta at the level of T10. The basilar artery (choice B) is formed by the joining of the vertebral arteries superior to the foramen magnum and thus does not provide blood supply to the spinal cord. Its branches supply the brainstem. The internal carotid artery (choice C) supplies the orbit via its ophthalmic branch and the brain by its anterior and middle cerebral branches. The posterior inferior cerebellar artery (choice D) is a branch of the vertebral artery, providing vascular supply to the medulla oblongata and the inferior aspect of the cerebellum.

• Question 102:

  Papez circuit provides the anatomic substrate for the integration of cognitive, emotional, and expressive brain activities. Which of the following brain structures participates in Papez circuit?

  A. amygdale
  B. caudate nucleus
  C. mammilary nuclei
  D. suprachiasmatic nucleus
  E. ventrolateral thalamic nucleus
  C. mammilary nuclei

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  Explanation

  Section: Anatomy Papez circuit is formed by efferent fibers of the hippocampus, which project by way of the fornix to the mammillary nuclei in the hypothalamus. Axons from the mammillary nuclei project to the anterior thalamic nucleus, which is connected to the cingulate gyrus. The latter projects back to the hippocampus by way of the parahippocampal gyrus. Papez circuit integrates cortical, hypothalamic, and thalamic functions resulting in the integration of cognitive, emotional, and expressive brain functions. The ventrolateral thalamic nucleus (choice E) is involved in motor pathways, being connected to the deep cerebellar nuclei and the motor cortex. The caudate nucleus (choice B) is a component of the basal ganglia and the extrapyramidal motor control system. Although the amygdala (choice A) is involved in emotional brain functions, it is not part of Papez circuit. The suprachiasmatic nucleus (choice D) is located in the hypothalamus and mediates the circadian rhythm.

• Question 103:

  The etiological agent of the graduate student's disease is usually transmitted by the ingestion of which of the following?

  A. cysts
  B. gametocytes
  C. larvae
  D. ova
  E. sporozoites
  A. cysts

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  Explanation

  Section: Microbiology/Immunology Cysts are the infective form of E. histolytica and are transmitted by ingestion of infected food or drink. The gametocyte (choice B) is the infective stage of malaria parasites for mosquitoes. Larval penetration of the skin is the mode of transmission for hookworms, Strongyloides and Schistosoma (choice C). Ingestion of ova (choice D) is the mode of transmission of such organisms as Enterobius vermicularis, Ascaris lumbricoides, Trichuris trichiura, and others, but not E. histolytica. Sporozoites (choice E) are the infective stage of malaria parasites spread to humans by mosquito bites.

• Question 104:

  A 26-year-old woman complains of the acute onset of anuria, purpura, and mental confusion. Her peripheral blood film displays marked thrombocytopenia and abundant schistocytes. Laboratory studies reveal elevations of bilirubin, creatinine, and lactose dehydrogenase. A skin biopsy shows numerous intravascular thrombi within the dermal microvasculature. What is the most likely diagnosis?

  A. acute idiopathic thrombocytopenia purpura
  B. BernardSoulier syndrome
  C. Glanzmann thrombasthenia
  D. MayHegglin anomaly
  E. thrombotic thrombocytopenic purpura
  E. thrombotic thrombocytopenic purpura

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  Explanation

  Section: Pathology and Path physiology Thrombotic thrombocytopenic purpura is an acute microangiopathic hemolytic anemia. The clinical picture usually includes mental alterations, anuria, mucosal bleeding, and purpura. An abnormal platelet- aggregating substance is the likely initiating event. Acute idiopathic thrombocytopenia (choice A) does not have a hemolytic component, lacks renal failure, and does not display thrombi in the skin biopsy. Bernard- Soulier syndrome (choice B) and Glanzmann thrombasthenia (choice C) are hereditary disorders of platelet aggregation. Clinical symptoms of a coagulopathy usually occur in infancy. MayHegglin anomaly (choice D) is an inherited condition with thrombocytopenia and morphologically abnormal WBCs. Hemolysis, acute onset, and mental aberrations do not typify this disorder.

• Question 105:

  Hemochromatosis, a disorder that is the result of excess iron accumulation, is caused by deficiencies in which of the following proteins?

  A. divalent metal transporter-1 (DMT1)
  B. human leukcocyte antigen (HLA) complex iron protein (HFE)
  C. ferritin
  D. ferroportin
  E. transferrin
  B. human leukcocyte antigen (HLA) complex iron protein (HFE)

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  Explanation

  Section: Biochemistry Hemochromatosis is defined as a disorder in iron metabolism that is characterized by excess iron absorption, saturation of iron-binding proteins, and deposition of hemosiderin in the tissues. The primary affected tissues are the liver, pancreas, and skin. Iron deposition in the liver leads to cirrhosis and in the pancreas causes diabetes. The excess iron deposition leads to bronze pigmentation of the organs and skin. In fact, the bronze skin pigmentation seen in hemochromatosis, coupled with the resulatant diabetes lead to the designation of this condition as bronze diabetes. The primary cause of hemochromatosis is the inheritance of an autosomal recessive allele. The locus causing hemochromatosis has been designated HFE and is a major histocompatibility complex (MHC) class-1 gene. The gene encodes an alpha-chain protein with three immunoglobulinlike domains. This alpha- chain protein associates with beta-2- microglobulin. Normal HFE has been shown to form a complex with the transferrin receptor and in so doing is thought to regulate the rate of iron transfer into cells. A mutation in HFE will therefore, lead to increased iron uptake and storage. The majority of hereditary hemochromatosis patients harbor a mutation in HFE that results in the substitution of Cys 282 for a Tyr. This mutation causes loss of conformation of one of the immunoglobulin domains in HFE. Another mutation found in HFE causes a change of His68 to Asp. DMT1 (choice A) is responsible for the intestinal absorption of dietary iron and thus, a deficiency in this protein would lead toreduced total body iron, not excess. Ferritin (choice C) is the intracellular iron binding and storage protein. A deficiency in ferritinwould lead to less iron deposition, not excess. Ferroportin (choice D) is the iron transport protein that moves dietary iron from within intestinal enterocytes across the basolateral membrane to the circulation, therefore, a deficiency in this protein would have consequences similar to deficiencies in DMT1. Transferrin (choice E) is the iron transport protein of the blood that binds to the trasferrin receptor on cell surfaces, allowing cellular uptake of circulating iron. A deficiency in this protein would lead to less iron deposition not more.

• Question 106:

  A 25-year-old male patient comes to your office experiencing inability to swallow and speech difficulty. The patient was in perfect health prior to the consumption of home-canned green beans. Which of the following is the most likely method of treatment?

  A. administration of staphylococcal enterotoxin antiserum
  B. administration of trivalent botulinum antitoxin
  C. immunization withS. aureus enterotoxin toxoid
  D. penicillin administration
  E. placement of the patient in a hyperbaric oxygen chamber
  B. administration of trivalent botulinum antitoxin

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  Explanation

  Section: Microbiology/Immunology The patient described in this question is showing the typical symptoms of botulism, which is commonly caused by types A, B, or E C. botulinum toxin. Therefore, early administration of potent botulinum antitoxin- containing antibodies to toxins A, B, and E constitutes the most appropriate type of treatment. Administration of staphylococcal enterotoxin antiserum could have been helpful if the atient was suffering from staphylococcal food poisoning, but he is not (choice A). Immunization with staphylococcal enterotoxin toxoid (choice C), administration of penicillin (choice A), or placing this patient in a hyperbaric chamber (choice E) are all useless.

• Question 107:

  On November 6, a patient had the onset of an illness characterized by fever, chills, headache, cough, and chest pain. The illness lasted 1 week. On December 5, she had another illness very similar to the first, which lasted 6 days. She had no flu immunization during this period. Her Hl titer to swine flu virus is shown in below figure. (There were no laboratory errors in any of the tests.) Which of the following is the most valid conclusion?

  

  A. It is impossible to relate either illness with the specific virus.
  B. The patient was ill with swine flu later, and the November 6 illness was due to another pathogen.
  C. The patient was ill with swine flu on December 20.
  D. The patient was ill with swine flu on November 6.
  B. The patient was ill with swine flu later, and the November 6 illness was due to another pathogen.

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  Explanation

  Section: Microbiology/Immunology Influenza viruses are classified into three main groups (A, B, and C) based on proteins in the ribonucleoprotein capsid. Each virus can be further subdivided into various groups when the hemeagglutinin (HA) and NA glycoproteins are examined. So far, 15 subtypes of HA (H1H15) and 9 subtypes of NA have been identified, and these can be found in many different combinations for influenza virus types or strains. Influenza viruses also have the ability to attach to and to agglutinate various types of red blood cells, including chicken RBCs. Antibodies made against HA antigens will neutralize HA activity and inhibit HA. This HI data can be used clinically to determine whether the same or different flu viruses have caused single or multiple outbreaks. In our clinical episode in this question and below figure, an HI titer of 10 (or 1:10 dilution of the patient's serum) represents little or no specific antibody against the specific virus, (swine flue virus) used in the HI test. Infection may have occurred in the past, with only small amounts of specific antibody remaining. The 160 (1:160) titer indicates a large amount of specific anti- swine flu immunoglobulins are now present. Apositive diagnosis for swine flu infection by December 20 is stated because it is fourfold or higher than the November 30 serum sample. As a result, choice B is the proper interpretation of these serological test results. Choice A is incorrect because we have specifically measured antibodies against swine flu on December 20. Choice C is partially correct but does not interpret results for November 6 and 30. Choice D is specifically incorrect since only swine flu was used in this HI test and the November 6 data is essentially negative.

• Question 108:

  If the DNA strand shown below is used as a template for RNA polymerase, what would be the sequence of the resultant mRNA following transcription?

  5'-CATTCCATAGCATGT-3'

  A. 5'-ACAUGCUAUGGAAUG-3'
  B. 5'-CAUUCCAUAGCAUGU-3'
  C. 5'-GUAAGGUAUCGUACA-3'
  D. 5'-UGUACGAUACCUUAC-3'
  A. 5'-ACAUGCUAUGGAAUG-3'

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  Explanation

  Section: Biochemistry Transcription occurs in the 5'3' direction, and during this process RNA polymerase will move in the 3'5' direction relative to the template DNA strand. Therefore, the correct transcriptional product from the DNA template begins with its 5' ribonucleotide corresponding to the complementary deoxyribonucleotide of the 3' end of the template. None of the other RNA strands (choices B, C, D, and E) could be products generated from the template shown.

• Question 109:

  At a blood alcohol level of 200 mg/dL (0.2%), which of the following correctly describes the systemic elimination process for ethanol?

  A. constant clearance via liver, kidney, and lungs
  B. first-order elimination via pulmonary exhalation
  C. first-order elimination via renal excretion
  D. second-order elimination via biliary secretion
  E. zero-order elimination via hepatic metabolism
  E. zero-order elimination via hepatic metabolism

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  Explanation

  Section: Pharmacology At a blood alcohol level of 200 mg/dL, most individuals are grossly inebriated; a level of 80100 mg/dL (0.08- 0.1%) is considered the legal threshold for intoxication in most states. Ethanol is metabolized by alcohol dehydrogenase (and to a lesser extent by the microsomal ethanol oxidizing system) to acetaldehyde, primarily in the liver. At most ethanol concentrations, the metabolizing system is saturated. As a result, zeroorder kinetics are observed. A typical adult shows a constant elimination rate manifested as a decline in blood level of 1622 mg/dL/h. As blood alcohol levels drop below about 100 mg/dL, elimination has characteristics intermediate between zero and first order. Only at concentrations below 1 mg/dL is the elimination truly first order. Constant clearance (choice A) requires first-order elimination. Pulmonary excretion (choice B) of ethanol accounts for only a minor component of elimination. Breath analyzers are used to estimate the blood alcohol level in drivers suspected of driving under the

  influence of alcohol. Renal excretion (choice C) also accounts for only a minor component of systemic elimination of alcohol. Because of the small size of the ethanol molecule, most of the alcohol undergoing filtration at the glomerulus is reabsorbed. Biliary secretion (choice D) is not significant in ethanol elimination. At a concentration of 200 mg/dL, blood alcohol is eliminated at zero-order kinetics via hepatic alcoholic dehydrogenase.

• Question 110:

  A 67-year-old retiree was employed for many years in the plastics industry where he was exposed to vinyl chloride. This industrial exposure has increased his likelihood of developing which of the following?

  A. focal nodular hyperplasia
  B. hepatic adenoma
  C. hepatic angiosarcoma
  D. hepatic fibroma
  E. hepatocellular carcinoma
  C. hepatic angiosarcoma

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  Explanation

  Section: Pathology and Path physiology Environmental exposure to vinyl chloride is associated with the later development of hepatic angiosarcoma. Focal nodular hyperplasia (choice A) and hepatic fibroma (choice D) do not at present have well-defined antecedent environmental exposure histories. Hepatic adenomas (choice B) occur sporadically in the setting of exogenous steroid hormone usage. Hepatocellular carcinoma (choice E) is associated with cirrhosis, chronic viral hepatitis, and aflatoxin exposure.