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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 721:

  A 25-year-old man with acquired immune deficiency disease (AIDS) has malabsorption, chronic abdominal pain, low-grade fever, and nonbloody diarrhea. In fecal smears, large numbers of oocysts containing four sporozoites were observed. Which of the following organisms is most likely the etiological agent?

  A. Cryptosporidium parvum
  B. Entamoeba histolytica
  C. Giardia lamblia
  D. Microsporidia species
  E. Taenia solium
  A. Cryptosporidium parvum

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  Explanation

  Section: Microbiology/Immunology All five organisms are protozoa. There are two intestinal protozoa specifically associated with AIDS that can cause transient diarrhea in immunocompetent individuals, but can cause debilitating, and potentially life- threatening, chronic diarrhea in AIDS patients. These organisms are C. parvum with no effective drug therapy, and Isospora belli treated with folate antagonists, such as trimethoprim- sulfamethoxazole. Microsporidia may cause severe, persistent, watery diarrhea in AIDS patients, but they produce spores rather than oocysts (choice D). T. solium is the pork tapeworm which occasionally may cause diarrhea (choice E). E. histolytica and G. lamblia may cause diarrhea, but they are not closely associated with AIDS (choices B and C).

• Question 722:

  In this same patient, the pectineus muscle is also badly bruised. This muscle is innervated by which of the following?

  A. genitofemoral nerve
  B. obturator nerve
  C. peroneal division of the sciatic nerve
  D. saphenous nerve
  E. tibial division of the sciatic nerve
  B. obturator nerve

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  Explanation

  Section: Anatomy The pectineus muscle is part of the adductor muscles of the thigh, which are innervated as a group by the obturator nerve. The genitofemoral nerve (choice A) provides sensory innervation for a small area of the skin in the medial superior aspect of the thigh, and motor innervation to the cremaster muscle. The peroneal division of the sciatic nerve (choice C) innervates the skin and muscles in the peroneal and anterior comparments of the leg. The saphenous nerve (choice D) is purely a sensory nerve for the medial aspect of the leg and the foot. The tibial division of the sciatic nerve (choice E) provides sensory and motor innervations to the posterior compartment of the leg and the plantar foot.

• Question 723:

  Which of the following drugs is most dangerous in suicidal overdosage?

  A. bupropion
  B. citalopram
  C. fluoxetine
  D. imipramine
  E. phenelzine
  D. imipramine

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  Explanation

  Section: Pharmacology Imipramine and other tricyclics are the most dangerous antidepressants when taken in overdosage because of their cardiotoxic (arrhythmias) and neurologic (seizures) effects. Bupropion (choice A), the selective serotonin reuptake inhibitors (SSRIs) (choices B and C), and MAO inhibitors (choice E) are considerably less dangerous.

• Question 724:

  In the coronal section of the head shown in following figure arrow 2 points to a structure that belongs to which of the following?

  

  A. ethmoid bone
  B. inferior nasal concha
  C. maxillary bone
  D. nasal septum
  E. vomer bone
  A. ethmoid bone

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  Explanation

  Section: Anatomy Arrow 2 points to the middle nasal concha, which is part of the ethmoid bone. Arrow 3 points to the ethmoid sinus, which also belongs to the ethmoid bone. The inferior nasal concha (arrow 1) is a separate bone (choice B). The maxillary bone (choice C) is located more laterally and does not contribute to the nasal conchae. The maxillary sinus, a part of the maxillary bone, is indicated by arrow 4. The vomer bone (choice E) forms part of the nasal septum (choice D), which can be seen here as the thin line between the nasal conchae. The nasal conchae have no attachment to the septum.

• Question 725:

  A 17-year-old man who reports to his physician that he is incapable of obtaining an erection is also quite embarrassed by the apparent enlargement of his breasts (gynecomastia). These symptoms, when present in males, are associated with an excessive production of which of the following hormones?

  A. corticotropin-releasing hormone
  B. GnRH
  C. growth hormone
  D. melanocyte-stimulating hormone
  E. prolactin
  E. prolactin

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  Explanation

  Section: Biochemistry Prolactin is necessary for initiation and maintenance of lactation. Physiologic levels act only on breast tissue primed by female sex hormones. Endocrine dysfunction leading to excessive prolactin production is associated with breast enlargement and impotence in males. Excessive production of corticotropin- releasing hormone (choice A) would result in an increase in adrenocorticotropic hormone (ACTH) production, which would lead to enhanced glucocorticoid and mineralocorticoid production. Excessive production of GnRH (choice B) would lead to increased production of luteinizing hormone (LH), follicle- stimulating hormone (FSH), and chorionic gonadotropin (hCG), with consequent effects on the female reproductive system. Excessive production of growth hormone (choice C) leads to gigantism if it occurs prior to epiphysial plate closure. If excessive release occurs following epiphysial plate closure acromegaly results, with characteristic facial changes (protruding jaw, enlarged nose) and enlarged feet,

  hands, and skull. Excessive production of melanocyte-stimulating hormone (choice D) would lead to hyperpigmentation of the skin.

• Question 726:

  A 74-year-old man dies after a 4-day hospital course with a clinical diagnosis of adult respiratory distress syndrome. At autopsy a pathologic diagnosis of diffuse alveolar damage is rendered. Which of the following microscopic findings of the lung tissue is most expected at autopsy?

  A. alveolar hyaline membrane formation
  B. eosinophilic inflammatory infiltrates
  C. hemorrhagic infarction
  D. pleural effusion and fibrous pleuritis
  E. pulmonary vasculature occluded by microthrombi
  A. alveolar hyaline membrane formation

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  Explanation

  Section: Pathology and Path physiology Hyaline membranes are usually seen microscopically in lungs showing diffuse alveolar damage. Grossly, the lungs are heavy, wet, and meaty. The clinical course is termed adult respiratory distress syndrome and is characterized by relative unresponsiveness to oxygen therapy. Eosinophilic inflammatory infiltrates (choice B), hemorrhagic infarction (choice C), pleural effusions (choice D), and vascular microthrombi (choice E) are not typically seen with diffuse alveolar damage.

• Question 727:

  A 52-year-old woman presents to her primary care physician complaining of increasing fatigue and mild shortness of breath. Blood work reveals a hypochromic anemia with a hemoglobin concentration of 10.4 g/ dL, MCV of 76 / , MCHC of 29 g/dL, and a decrease in the absolute reticulocyte count.

  

  WBC and platelet counts are within normal limits. Serum iron and ferritin levels are low and total iron- binding capacity is elevated. Which of the following conditions best accounts for these findings?

  A. anemia of chronic disease
  B. aplastic anemia
  C. hypothyroidism
  D. iron deficiency
  E. pernicious anemia
  D. iron deficiency

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  Explanation

  Section: Pathology and Path physiology Laboratory findings of a hypochromic anemia with a decrease in MCV and MCHC indices and a decreased reticulocyte count suggest a hypoproliferative anemia. The various microcytic, hypochromic anemias belong to this category and a partial differential diagnosis for these includes iron-deficiency anemia, anemia of chronic disease (ACD) (choice A), thalassemias, and sideroblastic anemias. ACD (choice A) may be distinguished from irondeficiency anemia by additional iron studies, as were performed in this case. While serum iron and ferritin levels are low and total ironbinding capacity (TIBC) is elevated in irondeficiency anemia, in ACD ferritin is typically normal or elevated, iron is low, and TIBC is low. ACD is associated with a functional, not an absolute, deficiency in iron as a result of sequestration by reticuloendothelial cells. Aplastic anemia (choice B) causes a pancytopenia, which was not the case in this patient as WBC and platelet levels were normal. Hypothyroidism (choice C) and pernicious anemia (choice E) are megaloblastic anemias, which demonstrate increased, not decreased, MCV values (~ >110 fL).

• Question 728:

  A 72-year-old male patient has suffered a cerebral infarct affecting the left cerebral hemisphere. On examination of the patient, it is noticed that his verbal output is fluent and paraphasic. His comprehension of speech is normal but repetition is severely impaired. Naming is also impaired, although when given a list, the patient is able to select the correct name. Special consultation with a neurologist results in a diagnosis of conduction aphasia. Which of the following brain structures is affected?

  A. arcuate fasciculus
  B. Broca's area
  C. nucleus ambiguous
  D. red nucleus
  E. Wernicke's area
  A. arcuate fasciculus

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  Explanation

  Section: Anatomy The arcuate fasciculus connects Wernicke's area to Broca's area, integrating comprehension of speech with motor speech. A lesion in this connecting bundle results in this unusual condition of conduction aphasia, characterized by impairment of repetition and naming. Broca's area (choice B) is the motor speech area and lesion in this area will result in motor speech impairment. However, this patient's verbal output is fluent. Alesion in the nucleus ambiguus (choice C) will result in dysarthria and also loss of a fluent verbal output. The red nucleus (choice D) is a mesencephalic motor nucleus, which does not participate in the central pathways for speech. Wernicke's area (choice E) is the speech comprehension area, which is intact in this patient because he comprehends spoken language.

• Question 729:

  In the central auditory pathways, second-order neurons are located in which of the following?

  A. cochlear (spiral) ganglion
  B. cochlear nuclei
  C. inferior colliculi
  D. nuclei of lateral lemniscus
  E. superior olivary nuclei
  B. cochlear nuclei

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  Explanation

  Section: Anatomy Second-order neurons in the central auditory pathways are located in the dorsal and ventral cochlear nuclei. They receive afferents from the first-order neurons located in the cochlear (spiral) ganglion (choice A). Secondorder fibers from the cochlear nuclei project in turn to the inferior colliculi (choice C), nuclei of lateral lemniscus (choice D), and superior olivary nuclei (choice E).

• Question 730:

  After isolating a pathogen from a patient, you monitor its growth in rich medium. According to below figure the rate of growth of the bacterial culture for which it was obtained reaches its maximum rate of growth between which of the following time periods?

  

  A. noon and 1:00 p.m.
  B. 2:00 p.m. and 3:00 p.m.
  C. 3:00 p.m. and 4:00 p.m.
  D. 3:00 p.m. and 5:00 p.m.
  E. 4:00 p.m. and 5:00 p.m.
  B. 2:00 p.m. and 3:00 p.m.

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  Explanation

  Section: Microbiology/Immunology By definition, the rate of growth of bacteria represents the change in the bacterial cell numbers over the change in time. From the choices given, the maximum rate of growth occurs between 2 p.m. and 3 p.m., where within 1 hour the number of bacteria has increased approximately threefold. Between noon and 1 p.m., 34 p.m., and 35 p.m., there is no increase in the number of cells and the rate of growth is zero.