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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 391:

  Antibiotic therapy is a critical step in the proper management of many patients infected with various types of bacteria. Infection with which microorganism listed below would not benefit from even prompt antimicrobial treatment?

  C. tetani
  B

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  Explanation

  Section: Microbiology/Immunology Botulism is worldwide in distribution and is caused by C. botulinum (choice B). C. botulinum is an anerobic, spore-producing grampositive organism. During the growth of the vegetative form of the organism, toxins (types AG) are liberated into the environment, especially when the bacteria undergo autolysis. Illness in humans is most often caused by ingestion of foods contaminated with the toxin. Botulism toxin is absorbed from the gut and binds to receptors of presynaptic membranes of motor neurons of the peripheral nervous system and cranial nerves. This inhibits the release of acetylcholine, producing a flaccid paralysis. Use of antibiotics would be inappropriate and useless since no living bacteria are involved in this presentation of disease. Use of antitoxins would be the method of choice for intervention. B. anthracis (choice A) causes anthrax by bacterial spores entering small abrasions in oral mucosa and germinating into vegetative bacteria. Antibiotics would be the treatment of choice in a clinical situation, although vaccines are available for domestic animals. The other three choices (C, D, and E) all include clostridial organisms that cause diseases where antibiotic treatment is necessary.

• Question 392:

  A patient suffering from Charcot-Marie-Tooth disease displays progressive degeneration of peripheral nerves, distal muscle weakness and atrophy, and defects in deep tendon reflexes. This condition is associated with an abnormal mutation in the gene encoding connexin-32. Connexins are normally found in which type of cell junctions?

  A. communicating (gap) junction
  B. hemidesmosome
  C. macula adherens (spot desmosome)
  D. occluding (tight) junction
  E. zonula adherens (belt desmosome)
  A. communicating (gap) junction

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  Explanation

  Section: Anatomy Communicating (gap) junctions are formed by connexins, which associate together in groups of six to form connexons. The alignment of connexions between two cells allows for direct channels of communication between their cytoplasms, facilitating the transfer of molecules such as calcium or cyclic adenosine monophosphate (cAMP). Hemidesmosome (choice B), macula adherens (spot desmosome; choice C), and zonula adherens (belt desmosome; choice E) are classified as anchoring junctions. They are associated with intermediate filaments (hemidesmosome and macula adherens) or with actin microfilaments (zonula adherens), but not connexins. Occluding (tight) junctions (choice D) contain the proteins occludin and claudin but not connexin.

• Question 393:

  A 57-year-old man presents with angina. He uses 1 or 2 nitroglycerin tables sublingually when he experiences chest pain. Which of the following drugs may interact to cause serious hypotension if taken 6 hours or less before the nitrate?

  A. clonidine
  B. diltiazem
  C. metronidazole
  D. phentolamine
  E. sildenafil
  E. sildenafil

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  Explanation

  Section: Pharmacology Nitrates release nitric oxide in smooth muscle and cause relaxation by facilitating the production of cGMP. Sildenafil (and related drugs for erectile dysfunction, tadalafil and vardenafil) inhibit the inactivation of cGMP. When sildenafil and a nitrate are combined, excessive amounts of cGMP accumulate and may cause severe hypotension. The other drugs listed (choices Athrough D) have no significant interactions with nitrates.

• Question 394:

  A2-month-old girl with a history of persistent physiologic jaundice following birth is now reported by her parents to sleep excessively and display little activity. Physical examination reveals abnormal deep tendon reflexes, hypothermia, and muscular hypotonia. Based on this information, which of the following would be the best therapy for her condition?

  A. antibiotics
  B. growth hormone
  C. thiamine
  D. thyroxine
  E. vitamin D
  D. thyroxine

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  Explanation

  Section: Pathology and Path physiology This infant has cretinism, a condition caused by a neonatal lack of thyroxine. Thyroid agenesis, iodine deficiency, ingestion of goitrogens, and hereditary enzymatic deficiencies may all result in a relative lack of biologically active thyroxine. Affected children may display lethargy, jaundice, hypothermia, muscular hypotonia, and mental retardation. Medicinal replacement of thyroxine is therapeutic. The mental retardation may not be reversible, however, unless treated early. Antibiotics (choice A) have no effect in treating cretinism. Growth hormone (choice B) is an effective treatment for pituitary dwarfism but it has no benefit in cretinism. Treatment with thiamine (choice C) is the appropriate therapy for beriberi, not cretinism. A lack of vitamin D (choice E) causes rickets, not cretinism.

• Question 395:

  In patients with type 4 renal tubular acidosis (RTA) aldosterone deficiency is often a prominent finding. Distal tubular transport of which of the following ions will be affected in these patients?

  A. hydrogen and potassium in exchange for sodium
  B. hydrogen only
  C. potassium only
  D. sodium and bicarbonate
  E. sodium only
  A. hydrogen and potassium in exchange for sodium

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  Explanation

  Section: Physiology The RTArefers to related conditions that are disorders of urine acidification even though other renal functions are not impaired. In type 4 RTA, distal nephron dysfunction is due to either inadequate aldosterone production or aldosterone resistance resulting from intrinsic renal disease. Thus, patients develop hyperchloremic acidosis with hyperkalemia, due to impaired distal tubular secretion of both potassium and hydrogen ions. Treatment of patients is directed at controlling serum potassium. Choices B, C, D, and E are inconsistent with the known actions of aldosterone.

• Question 396:

  A30-month-old child presents with coarse facial features, corneal clouding, hepatosplenomegaly, and exhibiting disproportionate short-trunk dwarfism. Radiographic analysis indicates enlargement of the diaphyses of the long bones and irregular metaphyses, along with poorly developed epiphyseal centers. Other skeletal abnormalities typify the features comprising dystosis multiplex. The child's physical stature and the analysis of bone development indicate the child is suffering from which of the following disorders?

  A. Hunter syndrome
  B. Hurler syndrome
  C. Maroteaux-Lamy syndrome
  D. Morquio syndrome type B
  E. Sanfilippo disease type A
  B. Hurler syndrome

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  Explanation

  Section: Biochemistry Although multiorgan involvement, liver and spleen enlargement, and skeletal abnormalities are common to all the mucopolysaccharidotic (MPS) diseases, each encompasses features that allow for specific diagnosis. Hurler syndrome is characterized by progressive multiorgan failure and premature death. Hallmark features include enlargement of the spleen and liver, severe skeletal deformity, and coarse facial features (which are associated with the constellation of defects referred to as dystosis multiplex).

  The disease results from a defect in alpha-L-iduronidase activity, which leads to intracellular accumulations of heparin sulfates and dermatan sulfates. The accumulation of these GAGs (glycosaminoglycan) in Hurler syndrome patients severely affect development of the skeletal system leading, primarily, to defective long bone growth plate disruption. Hunter syndrome (choice A) hasfeatures similar to that of Hurler with a lack of corneal clouding. Additionally, symptoms progress slower, with onset of symptoms occurring between 2 and 4 years of age. Maroteaux-Lamy syndrome (choice C) encompasses symptoms similar to Hurler but with normal mental development. Morquio syndrome (choice D) comprises two related disorders, both of which are characterized by short-trunk dwarfism, fine corneal deposits, and a skeletal dysplasia (spondyloepiphyseal) distinct from other MPS. Sanfilippo syndrome (choice E) comprises four recognized types characterized by severe CNS degeneration with only mild involvement of other organ systems.

  Symptoms do not appear until 26 years of age.

• Question 397:

  A 34-year-old female patient is sneezing and has a runny nose and watery eyes every summer. Her physician is convinced that the patient is suffering from an allergy and performs some skin tests. The results of these tests are shown below:

  

  Which of the following test allergens is most likely to be inducing the patient's symptoms?

  A. cat dander
  B. house dust
  C. Kentucky blue grass
  D. mold
  E. pollen
  C. Kentucky blue grass

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  Explanation

  Section: Microbiology/Immunology Ms Y is suffering from allergic rhinitis. The diagnosis of this allergy is made on the basis of clinical symptoms and the performance of a skin test, as well as a radioallergosorbent test (RAST). In a skin test, a battery of potential allergens is injected separately subcutaneously, and the area of the wheal and flare reaction is measured. The allergen which induces the greatest wheal and flare reaction when compared to solvent is usually the one which causes the allergy. In this case injection of Kentucky blue grass yielded the biggest wheal and flare reaction, and thus this is the allergen which caused rhinitis in the patient. Allergens such as mold (choice D), cat dander (choice A), house dust (choice B), and pollen (choice E) gave wheal and flare reactions which were either below or slightly above the skin test values of the allergen solvent. Therefore, they are not likely to be causing allergy in the patient.

• Question 398:

  A 57-year-old obese woman has Type II (noninsulindependent) diabetes mellitus of 15 years duration. Because her hyperglycemia is not well controlled with diet and oral hypoglycemic agents, she selfadministers insulin by injection. She comes to her physician's office with a fever and a draining external ear infection; Pseudomonas aeruginosa is cultured from the drainage fluid. Which of the following insulin regimens most closely mimics insulin release from a normally functioning pancreas?

  A. prebreakfast and predinner injections of lente insulin plus an injection of regular insulin if blood glucose (measured by finger-stick) exceeds 150 mg/dL
  B. premeal injections of insulin lispro plus morning and evening injections of detemir insulin
  C. premeal injections of NPH insulin plus an injection of insulin lispro at bedtime
  D. post-meal injection of lente insulin plus a prebreakfast injection of regular insulin
  E. Post-meal injections of insulin glargine
  B. premeal injections of insulin lispro plus morning and evening injections of detemir insulin

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  Explanation

  Section: Pharmacology Anormally functioning endocrine pancreas provides a low basal level of circulating insulin and spikes of insulin release in response to the ingestion of food and subsequent elevation of blood sugar. In a diabetic, use of morning and evening injections of a very long-acting form such as lente, glargine, or detemir insulin provides a low basal level of circulating insulin. Premeal injections of rapid-acting insulin provide the spikes of circulating insulin needed to deal with the dietary glucose load. Regular insulin or insulin lispro, glulisine, or aspart is needed for this purpose. Insulin detemir and glargine are very long-acting recombinant insulins that are used to provide basal levels. They are of no value for controlling transient hyperglycemic spikes.

• Question 399:

  A 43-year-old man complains of a 25-lbs weight gain over the past 2 years distributed mostly in the trunk and face. He also states that he has developed abdominal stretch marks and facial acne during the same time period. Physical examination reveals BP 160/100 mm Hg and pulse 90 per minute. Significant laboratory tests were fasting glucose 145 mg/dL and 24-hour urinary free cortisol greatly increased at 390 g/d. The dexamethasone suppression test showed suppression of the urinary free cortisol at high but not low doses. Based upon this information, you would expect to find a hormonally- active tumor in which of the following sites?

  A. adrenal cortex
  B. adrenal medulla
  C. anterior pituitary
  D. posterior pituitary
  C. anterior pituitary

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  Explanation

  Section: Pathology and Path physiology This patient has the physical stigmata of excess cortisol or Cushing syndrome which is confirmed by the urinary free cortisol levels. Cortisol also has some mineralocorticoid activity producing sodium and water retention and hypertension. Cortisol promotes gluconeogenesis leading to glucose intolerance, hyperinsulinism and, in some patients, frank diabetes. The fact that urinary cortisol levels were suppressed by high but not low doses of dexamethasone strongly suggests a pituitary-based Cushing syndrome due to excess ACTH production or Cushing disease. ACTH-producing cells or corticotropes are found in the anterior pituitary. The dexamethasone suppression test rules out the adrenal (choices Aand B) as a site for the tumor and posterior pituitary (choice D) is not the location of the ACTH- producing cells.

• Question 400:

  A 13-year-old boy with severe asthma has been treated for 6 months with good control of nocturnal wheezing. His parents now complain that he seems to be very fat around the face and shoulders, bruises easily, and has discolored streaks on his abdomen. Laboratory tests reveal moderate hyperglycemia. He has probably been receiving which of the following drugs?

  A. ipratropium
  B. prednisone
  C. salmeterol
  D. terbutaline
  E. theophylline
  B. prednisone

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  Explanation

  Section: Pharmacology This patient's presentation is typical of glucocorticosteroid toxicity (iatrogenic Cushing syndrome). Prednisone is an effective corticosteroid frequently used by mouth to control severe asthma that is not responsive to inhaled corticosteroids. High doses of prednisone, given chronically, very predictably cause signs and symptoms of hypercorticism. Ipratropium (choice A) is an antimuscarinic agent used in asthma by inhalation. Salmeterol (choice C) is a long-acting beta-2-selective agonist used by inhalation.

  Terbutaline (choice D) is a shortacting beta-2-selective agonist, also used by inhalation. Theophylline (choice E) is an oral drug that causes CNS stimulation (including convulsions in toxic overdosage), but not hypercorticism.