USMLE USMLE Certifications USMLE-STEP-2 Questions & Answers

• Question 381:

  A group of male workers between the ages of 20 and 39 years are being screened for lung disease by spirometry. Nine subjects are examined. Their forced expiratory volume in 1 second (FEV1) divided by forced vital capacity (FEV1/ FVC %) results are 80, 76, 73, 61, 64, 79, 64, 64, and 78. What is the range?

  A. 17

  B. 18

  C. 19

  D. 20

  E. 21

  Correct Answer: C

  The range is the difference between the highest and lowest readings in a group of observations. The lowest reading is subtracted from the highest. Only one of these two readings is included in the range. If both were to be included, the range would be 20, as there are 20 possible values included in the limits of the data presented in the question. For simplicity, the first method is generally accepted. One disadvantage of the concept of range is that it may increase as the number of observations increases.

• Question 382:

  A group of male workers between the ages of 20 and 39 years are being screened for lung disease by spirometry. Nine subjects are examined. Their forced expiratory volume in 1 second (FEV1) divided by forced vital capacity (FEV1/ FVC %) results are 80, 76, 73, 61, 64, 79, 64, 64, and 78. What is the median value?

  A. 61

  B. 64

  C. 71

  D. 73

  E. 76

  Correct Answer: D

  The median is the observation that lies in the middle of the series, if the observations are tabulated in numerical order. Half of the observations are lower in numerical value than the median, and the remainder are higher. Clearly, this value is easily identified if the series contains an odd number of readings. Although not very frequently used as a statistic, the median has the advantage of not being affected by extreme observations. For example, if the lowest reading of those in the question had been 55 instead of 61, the median would be unchanged.

• Question 383:

  A group of male workers between the ages of 20 and 39 years are being screened for lung disease by spirometry. Nine subjects are examined. Their forced expiratory volume in 1 second (FEV1) divided by forced vital capacity (FEV1/ FVC %) results are 80, 76, 73, 61, 64, 79, 64, 64, and 78. What is the modal reading?

  A. 61

  B. 64

  C. 71

  D. 73

  E. 76

  Correct Answer: B

  The modal reading is the most frequently occurring observation in a group of data. (On a graph of frequency of different observations, this would be where the peak is.) This reading generally is not very useful for statistical calculations, and when the series is small, no modal reading may even occur. When drawn in graph form, the determination of unimodal or bimodal distribution may be a helpful concept from which conclusions may be drawn. The distribution in the example that accompanies the question is unimodal.

• Question 384:

  A group of male workers between the ages of 20 and 39 years are being screened for lung disease by spirometry. Nine subjects are examined. Their forced expiratory volume in 1 second (FEV1) divided by forced vital capacity (FEV1/ FVC %) results are 80, 76, 73, 61, 64, 79, 64, 64, and 78. What is the mean?

  A. 61

  B. 64

  C. 71

  D. 73

  E. 76

  Correct Answer: C

  The mean, or average, is the total of all readings divided by the number of readings, or (x)/n. In the example described in the question, the total of all readings equals 639; the number of readings is 9. Therefore, the mean is 639 divided by 9, or 71.

• Question 385:

  Several members of a group of young adults camping, cooking, and traveling together in the developing world develop fever, malaise, nausea, and vomiting, and have dark urine. Two have yellow sclera. How could this best have been prevented?

  A. avoiding eating local foods

  B. washing hands before eating

  C. taking prophylactic Pepto Bismol

  D. getting vaccinations

  E. cooking all foods thoroughly and drinking boiled water

  Correct Answer: D

  Hepatitis Awas first isolated in 1973. Since then, it has been demonstrated to be conveyed from person to person chiefly by the fecal-oral route. Humans appear to be the only natural host. Outbreaks attributed to food and water supplies are frequently reported. There were an estimated 61,000 new infections in the United States (particularly in the West) in 2003, continuing a trend downward. It does not cause chronic infection. Vaccination is effective and available for persons at risk, including travelers. Examples of where outbreaks occur include: among travelers, among young adults clustered together, and in day care environments.

• Question 386:

  Six hundred asymptomatic men with prostatic nodules are given a PSA test to screen for prostate cancer. With a cutoff of 5 ng/mL, the PSA results are positive in 200 cases. Of these, 100 persons are confirmed

  on biopsy and follow-up testing to have the disease; however, 50 of the individuals who had negative test

  results are also shown to have the disease based on biopsy. You have been asked to screen another

  similar group of 50 men using the PSA test.

  What proportion of persons with no prostatic cancer will you correctly identify in the new group?

  A. 16.7%

  B. 22.2%

  C. 66.7%

  D. 77.8%

  E. 87.5%

  Correct Answer: D

  The percentage specificity of a test is the percentage of those persons without the disease (true negatives [TN]) who were correctly labeled by the test as not diseased. The false positives (FP) are those who were incorrectly labeled by the test as having the disease. The specificity is thus expressed as TN/(TN + FP) .

  100. In the example given in the question, the calculation is 350/(350 + 100) x 100 = 77.8%

• Question 387:

  Six hundred asymptomatic men with prostatic nodules are given a PSA test to screen for prostate cancer.

  With a cutoff of 5 ng/mL, the PSA results are positive in 200 cases. Of these, 100 persons are confirmed

  on biopsy and follow-up testing to have the disease; however, 50 of the individuals who had negative test

  results are also shown to have the disease based on biopsy. You have been asked to screen another

  similar group of 50 men using the PSA test.

  What proportion of persons having prostatic cancers will you correctly identify in the new group?

  A. 16.7%

  B. 22.2%

  C. 66.7%

  D. 77.8%

  E. 87.5%

  Correct Answer: C

  Several concepts are important in determining the sensitivity of tests. The percentage sensitivity is the percentage of individuals with the disease (true positives [TP]) detected by the tests. False negatives (FN) are those who have the disease, but who were not detected by the test. The percentage sensitivity is calculated as: TP/(TP + FN) . 100. In the example given in the question, the calculation is 100/(100 + 50) x 100 = 66.7%.

• Question 388:

  For which of the following circumstances would you consider initiating chelation, such as with CaEDTA, to treat workers for workplace exposure to lead and other heavy metals?

  A. as prophylaxis for all employees exposed to metal dust and fumes

  B. only after waiting for toxic effects of heavy metal exposure to resolve on their own

  C. only for patients with symptomatic disease

  D. when metal is being absorbed through the gastrointestinal (GI) tract

  E. when patients remain in a workplace where exposure occurs

  Correct Answer: C

  Chelating drugs are given as treatment for symptomatic poisoning by lead and other heavy metals. They should not be given prophylactically, since the agents themselves have some possible toxic side effects. These toxic effects may add to those already caused by ingestion of the metals and may actually increase absorption of the metal. For these reasons, advice should be given to workers to seek employment away from exposure to the offending agent while therapy continues. Removal from exposure is essential if levels are high.

• Question 389:

  A 4-year-old child presents to his pediatrician with a complaint of a mild rash and fevers. His travel history is positive for a camping trip 2 weeks prior. The parents do not recall a tick bite on the child and do not remember if there were ticks in the area. No other members of the family complain of similar symptoms and the child does not attend day care. Physical examination is positive for a temperature of 100.9°F with an erythematous rash noted over the child's trunk. Which of the following factors would significantly increase the possibility that the child has Lyme disease?

  A. The camping trip occurred in an area endemic for the tick Ixodes dentatus.

  B. The camping trip occurred in Texas.

  C. The camping trip occurred in an area with a high indigenous population of lizards.

  D. The camping trip occurred in an area endemic for the white-footed mouse.

  E. The camping trip occurred in an area which had recently been sprayed with pesticides.

  Correct Answer: D

  The principal risk factor for Lyme disease in the United States is residence in an area with high infestation rates of infected ticks. In the coastal northeastern United States, the whitefooted mouse is the most competent vertebrate reservoir of B. burgdorferi, the organism responsible for Lyme disease, with infection rates of up to 80%. Therefore, residence in an area where there is a large population of whitefooted mice will also increase the likelihood ofexposure to Lyme disease. The tick species I. dentatus has maintained cryptic cycles of B. burgdorferi, but it is not considered to be a public health risk because it rarely feeds on humans. Texas is not one of the endemic areas for Lyme disease. Lizards are actually considered to be a zooprophylactic host because they are an incompetent reservoir of B. burgdorferi, and they are also a preferential feeding source for immature stages of the tick. Pesticide spraying is actually protective against tick infestation and therefore will reduce the chances of tick exposure.

• Question 390:

  The causative organism of cholera, Vibrio cholerae, was first isolated by Koch in 1883. There have been seven pandemics of cholera, with the most recent subsiding only in the 1980s. With humans as the usual reservoir, the organism spreads as man travels. Which of the following is the most likely mode of spread?

  A. contaminated fomites

  B. specific strains of mosquitoes

  C. food cleaned in contaminated water

  D. person-to-person transmission via inhalation of droplet nuclei in crowded places

  E. the phlebotomine fly (sand fly)

  Correct Answer: C

  Humans are the usual reservoir of V. cholerae. It tolerates exposure and drying poorly. It survives longest in water, especially if the water is at temperatures of 1823°C (6070°F). It does not spread on infecte d clothing (fomites). Direct person-to-person transmission probably does not occur. Contaminated water is the main source of infection (e.g., frequent exposure to polluted surface water through bathing, food preparation, and utensil washing). Although flies may transport small numbers of vibrios from excreta to food, lack of multiplication makes it unlikely that flies play an important part in transmission. Mosquitoes are not vectors.