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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 191:

  Following an automobile accident a patient suffers a pelvic fracture and significant internal blood loss resulting in hemorrhagic shock. Which of the following organs has the largest specific blood flow (blood flow per gram of tissue) under resting conditions and is especially vulnerable during the shock phase?

  A. brain
  B. heart muscle
  C. kidneys
  D. skeletal muscle
  E. skin
  C. kidneys

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  Explanation

  Section: Physiology During resting conditions, approximately 15% of the cardiac output goes to the brain, 15% to the muscles, 30% to the GI tract, and 20% to the kidneys. However, when normalized by organ weight, the kidneys receive the largest specific blood flow (400 mL/100 g) at rest and are particularly vulnerable during hemorrhagic shock. The brain (choice A) also receives relatively high specific blood flow (50 mL/100 g). Heart muscle (choice B), not surprisingly, also has a relatively high resting specific blood flow (60 mL/100 g), which may increase fivefold during exercise. Skeletal muscles (choice D) have low specific blood flow (23 mL/100 g) at rest, which may increase up to 20-fold during strenuous exercise. Blood flow through the skin (choice E) varies between 1 and 100 ml/100 g and serves temperature regulation.

• Question 192:

  Taking a careful history of a 20-year-old female patient suffering from dissociative identity disorder is likely to reveal which of the following?

  A. a history of criminal behavior during childhood
  B. an excellent school record
  C. physical or sexual abuse in childhood
  D. setting of fires in childhood
  C. physical or sexual abuse in childhood

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  Explanation

  Section: Behavioral Science and Biostatics Dissociative identity disorder (multiple personality disorder) is often associated with a history of severe emotional, physical, or sexual abuse in childhood. The dissociation may serve an adaptive function that enables the patient to tolerate an intolerable situation. The patient's school record is often poor due to memory disturbance associated with the disorder. Choices A and D are associated with antisocial personality, and choice A is unlikely because dissociation often causes inattention and poor school grades.

• Question 193:

  You are studying the effects of a compound on the respiratory activity of isolated mitochondria. Your experiments demonstrate that oxygen consumption is normal when pyruvate and malate are added. Oxygen consumption is also seen to be normal when succinate is added. However, you find that the production of ATP is severely impaired in the presence of pyruvate/malate or succinate when the compound is added to the reaction. These results most closely resemble the effects that would be seen by the addition of which of the following inhibitors of oxidative phosphorylation?

  A. antimycin A
  B. azide
  C. dinitrophenol
  D. oligomycin
  E. rotenone
  C. dinitrophenol

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  Explanation

  Section: Biochemistry The normal flow of electrons, through the proteins of the oxidative phosphorylation machinery, is coupled with the establishment of a proton gradient across the inner mitochondrial membrane. The establishment of this pH gradient is the chemiosmotic potential that is coupled with the production of ATP. If the process of proton movement is uncoupled from the normal pathway through the ATP synthase complex, oxygen can be consumed but no ATP will be synthesized. This is the effect of uncoupling agents such as DNP, which act to discharge the proton gradient. Antimycin A (choice A) is an antibiotic that functions by inhibiting cytochrome b of complex III. Azide (choice B) inhibits cytochrome oxidase of complex IV. Oligomycin (choice D), a streptomyces antibiotic, inhibits ATP synthase. Rotenone (choice E) inhibits complex I.

• Question 194:

  After an abdominal surgery for removal of ovarian cysts, a 56-year-old female has had low-grade fever for the past 2 weeks. She has a history of rheumatic fever as a child. Three of the blood cultures grew gram- positive cocci. Which of the following is the most likely etiologic agent?

  A. Group A streptococci
  B. group B streptococci
  C. group C streptococci
  D. group D streptococci
  E. S. viridans
  D. group D streptococci

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  Explanation

  Section: Microbiology/Immunology There are at least 12 species of enterococci, and all are classified as group D streptococci (choice D). E. faecalis is the most common and causes 8590% of enterococcal infections. As long as enterococci remain in the intestinal tract, they are seldom involved in disease. Yet, they are extremely efficient opportunists. The enterococci are among the most frequent causes of nosocomial infections, especially in intensive care units. They are commonly found in complications of intra-abdominal surgery, often as part of a mixed culture of microorganisms. Antibiotic therapy (cephalosporins, etc) is often a selective feature since enterococci are resistant to many of the antimicrobials in common use. Group Astreptococci (choice A) would be represented by S. pyogenes and is the group which causes about 95% of human infections (skin, septicemia, organs, etc), but would seldom be found in intra-abdominal complications. Streptococci B (choice B) are most often found in infections of infants since type B streptococci are found routinely in the vaginal flora. Group C streptococci (choice C) usually occur in the nasopharynx and may cause sinus, blood, or heat infections. S. viridans (choice E) are usually found in the oral cavity and may be opportunists involving blood and heart infections.

• Question 195:

  A 79-year-old woman presents with a complaint of chronic and progressive mid-back pain for several months but no other symptoms. Spinal x-rays reveal two compression fractures of the T10 vertebral body. Calcium, phosphate, and alkaline phosphatase levels are all within normal range. Histological examination of the affected vertebral bone would most likely reveal which of the following?

  A. bony necrosis with acute and chronic inflammation and granulomas
  B. disorganized interconnecting trabeculae of woven bone rimmed by osteoblasts
  C. thickened, irregular trabeculae with prominent cement lines
  D. thinning of the cortical and trabecular bone
  E. widened osteoid seams of peripheral trabeculae
  E. widened osteoid seams of peripheral trabeculae

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  Explanation

  Section: Pathology and Path physiology This woman is suffering from senile, or postmenopausal, osteoporosis. In such patients, the calcium and phosphate levels will typically be within the reference range. The alkaline phosphatase will also usually be normal, but may be increased following fractures. Microscopic examination of affected bones will reveal loss of bone mass tending to affect horizontal more than vertical trabeculae. Bony necrosis with acute and chronic inflammation and granulomas (choice A) occurs in tuberculosis infection of the spine, where it is referred to as Pott disease. Disorganized interconnecting trabeculae of woven bone rimmed by osteoblasts (choice B) describes the appearance of an osteoblastoma, a benign bone tumor that is histologically very similar to and often described in conjunction with an osteoid osteoma. A characteristic difference between these two benign bone tumors is that osteoid osteomas are very painful lesions (excess prostaglandin E production) that respond dramatically to aspirin, while osteoblastomas produce a dull, achy pain that does not respond to aspirin. Thickened, irregular trabeculae with prominent cement lines (choice C) describe the morphology seen in Paget disease. Widened osteoid seams of peripheral trabeculae (choice E) reflect the appearance of unmineralized osteoid found in osteomalacia. Alkaline phosphatase levels are typically increased in osteomalacia (and rickets) and Paget disease.

• Question 196:

  Apatient is being treated for metastatic prostate cancer. Which of the following agents is used parenterally in prostate cancer to reduce luteinizing hormone (LH) release from the pituitary even though it initially increases LH release?

  A. diethylstilbestrol
  B. finasteride
  C. flutamide
  D. ketoconazole
  E. leuprolide
  E. leuprolide

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  Explanation

  Section: Pharmacology Leuprolide is a GnRH agonist that when given in pulsatile fashion stimulates pituitary LH release, but when given continuously, or in a depot preparation, suppresses LH release and reduces plasma testosterone levels to castration levels. Because it is a peptide, it must be given parenterally. It is used extensively in the treatment of metastatic prostate cancer. Diethylstilbestrol (choice A) is an oral nonsteroidal synthetic estrogen that is now rarely used in the treatment of prostate cancer. Use of this agent produces undesirable feminization and is reserved for cases unresponsive to other agents. Finasteride (choice B) is an orally active inhibitor of the 5-alpha-reductase that converts testosterone to dihydrotestosterone in the prostate. Because conversion of testosterone to dihydrotestosterone is essential for the androgenic effects in the prostate, inhibition of the 5-alpha-reductase decreases the stimulation of the prostate. Unfortunately, the drug has limited efficacy in prostate malignancy. Finasteride is used for the treatment of benign prostatic hyperplasia because it produces a moderate decrease in prostate size and improvement in urinary flow. It is also used for the treatment of androgenic hair loss. Flutamide (choice C) is a nonsteroidal antiandrogen occasionally used in the treatment of prostate cancer that is unresponsive to leuprolide. It is active orally. Ketoconazole (choice D) is an oral antifungal agent that has the property of inhibiting the cytochrome P450 isoforms involved in steroid biosynthesis. It has been used in the treatment of prostate cancer with mixed results.

• Question 197:

  Poliovirus type 2 has been isolated from the stool of a 55-year-old patient who has experienced malaise, fever, nausea, and vomiting. An infant grandchild was vaccinated about 3 weeks prior to onset of the disease.

  What should the laboratory do to determine whether the isolated virus is related to the vaccine strain or a wild-type virus?

  A. determine the cytopathic effects of the virus
  B. do neutralization studies using the infant's serum
  C. do oligonucleotide mapping of the wildtype and vaccine strains
  D. inoculate the virus into mice to determine whether it kills them
  E. stain the virus with fluorescent antibody
  C. do oligonucleotide mapping of the wildtype and vaccine strains

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  Explanation

  Section: Microbiology/Immunology To determine whether the poliovirus is a wild-type virus or if it is related to that used for vaccination, it will be necessary to prepare oligonucleotide maps of the isolated virus and compare them to those of the wild- type and vaccine strains. Many viruses may kill mice; thus, one cannot determine whether poliovirus strains are related or unrelated by mouse lethality studies (choice D). Cytopathology cannot be used for definitive diagnosis of poliovirus strains or other types of viruses (choice A). Viral neutralization assays using the grandchild's serum will only indicate whether the grandchild has been exposed to and vaccinated for the polioviruses in question, but will not establish whether the poliovirus isolated from the stool of the grandfather is related to the poliovirus used to vaccinate his grandchild (choice B). Staining of the virus with fluorescent antibody may detect the presence of poliovirus, but will not establish whether the poliovirus is a wild-type virus or if it is related to that used for vaccination (choice E).

• Question 198:

  A 47-year-old man has undergone a gradual personality change over the past 67 months with increasing moodiness and irritability During your examination you note some choreiform involuntary movements and also recognize some cognitive impairment. Afterward you talk to the man's brother who accompanied the patient to your office and he tells you that their father had similar problems for about a year prior to his death. If you could examine this patient's brain, which of the following would you expect to find?

  A. areas of demyelination of the white matter
  B. atrophy and loss of neurons of the caudate nucleus
  C. loss of pigmented neurons of the substantia nigra
  D. neurons ballooned with cytoplasmic vacuoles
  E. selective atrophy of anterior frontal and temporal lobes
  B. atrophy and loss of neurons of the caudate nucleus

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  Explanation

  Section: Pathology and Path physiology In Huntington's disease there is a symmetric atrophy of the caudate nuclei with a great reduction in the number of neurons. The putamen is also affected but usually less so. Areas of demyelination of white matter (choice A) are the classic finding in multiple sclerosis. Loss of pigmented neurons of the substantia nigra (choice C) is a consistent change seen in Parkinson's disease. Neurons ballooned with cytoplasmic vacuoles (choice D) may be seen in a number of lysosomal storage diseases of which Tay- Sachs disease is the classic example. Selective atrophy of anterior, frontal, and temporal lobes (choice E) is seen in Pick disease. There is a severe atrophy of the frontal and temporal lobes which can be sufficient to produce the "knife-edge" appearance characteristic of this disease.

• Question 199:

  Which of the following substances is most likely to prevent the attachment of the mold allergen to the sensitized mast cells in this patient?

  A. complement
  B. corticosteroids
  C. cromolyn sodium
  D. epinephrine
  E. IgG anti-mold
  E. IgG anti-mold

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  Explanation

  Section: Microbiology/Immunology The IgG anti-mold-blocking antibody will combine with the mold allergen and will not permit the mold allergen to reach the IgE antimold on the surface of the mast cells, thus inhibiting the allergic reaction. Corticosteroids suppress inflammation, but will not prevent the attachment of the mold allergen to the sensitized mast cells by the patient (choice B). Complement is not fixed by IgE allergen complexes and plays no role in the attachment of mold allergen to sensitized mast cells (choice A). Epinephrine reverses constriction of bronchioles and bronchi, and also has no effect on the attachment of mold allergen to sensitized mast cells (choice D). Cromolyn sodium stabilizes the membrane of mast cells and thus prevents release of histamine from mast cells (choice C).

• Question 200:

  A 34-year-old male is brought into the emergency room, having been found unconscious in his apartment. Apparently, he has been in this state for 2 days. The laboratory values for the patient's blood and urine are consistent with elevated circulating antidiuretic hormone (ADH). Which one of the following will directly stimulate ADH in this patient?

  A. angiotensinogen
  B. extracellular fluid osmolality increase
  C. temperature decrease
  D. thyroid hormone
  E. volume increase
  B. extracellular fluid osmolality increase

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  Explanation

  Section: Physiology ADH acts to increase renal water conservation secondary to dehydration. Choice B, which can result from dehydration, is the logical trigger for ADH secretion. Angiotensinogen (choice A) is inactive and will not directly stimulate ADH, but its active metabolite angiotensin II or III will increase ADH secretion. Choices C, D, and E will either inhibit, or have minimal effects on ADH secretion.