USMLE USMLE Certifications USMLE-STEP-2 Questions & Answers

• Question 371:

  Workers in certain occupations are exposed to diseases for which animals are the reservoir. These

  workers may then become a source of infection to others.

  Park rangers are most likely to acquire and transmit the infectious disease?

  A. anthrax

  B. brucellosis

  C. Lyme disease

  D. murine (endemic) typhus

  E. salmonellosis

  Correct Answer: C

  Lyme disease was first comprehensively described in Connecticut in 1977. It is found in temperate North America and Eurasia. It may be identified some time after the initial illness, which may be a vague collection of symptoms. The vector is a tick found on the white-footed mouse, which is the reservoir. Persons who spend a significant amount of time in affected wooded areas are at risk for Lyme disease.

• Question 372:

  Workers in certain occupations are exposed to diseases for which animals are the reservoir. These workers may then become a source of infection to others.

  Livestock worker are most likely to acquire and transmit the infectious disease?

  A. anthrax

  B. brucellosis

  C. Lyme disease

  D. murine (endemic) typhus

  E. salmonellosis

  Correct Answer: B

  Brucella infections in humans follow a varied and sometimes chronic or recurrent course. Chronic disease is rare in appropriately treated patients. Human infections generally occur through one of three routes: ingestion, direct contact, or inhalation. Cattle sheds become infected after abortion, a manifestation of the disease, or occasionally after normal parturition.Worldwide, ingestion of unpasteurized dairy products is the primary source of infection.

• Question 373:

  Workers in certain occupations are exposed to diseases for which animals are the reservoir. These workers may then become a source of infection to others.

  Warehouse worker are most likely to acquire and transmit the infectious disease?

  A. anthrax

  B. brucellosis

  C. Lyme disease

  D. murine (endemic) typhus

  E. salmonellosis

  Correct Answer: D

  Rodents frequently infest granaries, despite strenuous efforts to control them. Murine (endemic) typhus has its principal reservoir in rodents and is conveyed to humans by flea bites. In the United States, murine or endemic typhus occurs mainly in the Southeastern and Gulf states. It is also common in parts of South America, Africa, and Southeast Asia. The disease generally follows a less severe course than epidemic typhus.

• Question 374:

  Select the ONE best lettered answer that is most closely associated with it. Each lettered answer may be

  selected once, more than once, or not at all.

  Workers in certain occupations are exposed to diseases for which animals are the reservoir. These

  workers may then become a source of infection to others.

  Butchers are most likely to acquire and transmit the infectious disease?

  A. anthrax

  B. brucellosis

  C. Lyme disease

  D. murine (endemic) typhus

  E. salmonellosis

  Correct Answer: A

  Cutaneous anthrax is associated with a characteristic skin lesion, which becomes infected by the introduction of the bacillus through the skin. Occasionally, an infected carcass is not identified before butchering. A minor wound on the butcher may become infected. Human anthrax is secondary to the disease in animals, primarily mammals. In humans, the disease has three major clinical forms: cutaneous, inhalational, and GI.

• Question 375:

  An investigator in a community hospital decides to examine all patients for a problem with alcoholism to determine prevalence in the community. In addition to recall bias, which of the following is the most obvious error in selecting all patients admitted to a community hospital?

  A. observer bias

  B. selection bias

  C. detection bias

  D. interpretive bias

  E. calculation bias

  Correct Answer: B

  Bias is a systematic error that may be introduced, generally unwittingly, into an investigation. Selection bias is due to systemic differences between those selected and not selected for a study. When selecting all admissions to a community hospital, alcoholics may be overrepresented among hospital patients and not reflect the community base from which the patients were admitted. Further, the investigator has not chosen a control sample but only patients already in the hospital, which further invalidates any outcome. While other biases may creep into this study, the selection bias is the major problem with the study.

• Question 376:

  A large segment of the population in your community smokes. In order to convince your patient population that it would be beneficial to quit smoking in order to prevent lung cancer, you conduct a casecontrol study of cigarette smoking and lung cancer. The results from that study can be found in the table below.

  

  Which of the following is most true concerning ORs and relative risk calculations, as pertaining to this study?

  A. The overall size of the series is too large to estimate relative risk.

  B. If the number of controls were increased, the two ratios would be similar.

  C. The number of controls is twice as many as the cases.

  D. The OR is not appropriate for this series.

  E. The disease is rare.

  Correct Answer: B

  For ease of calculation, to illustrate the concept, the numbers in the question were kept simple. If the numbers of controls were increased, the difference would be smaller. The ORs are a reasonable estimate when denominator data are missing. Relative risk is determined when denominator data are available. Few case-control experiments have denominator data.

• Question 377:

  A large segment of the population in your community smokes. In order to convince your patient population that it would be beneficial to quit smoking in order to prevent lung cancer, you conduct a casecontrol study of cigarette smoking and lung cancer. The results from that study can be found in the table below.

  

  Using the same table, what is the relative risk?

  A. 3

  B. 6

  C. 9

  D. 12

  E. cannot be calculated

  Correct Answer: E

  A true risk ratio cannot be calculated from a case-control study.

• Question 378:

  A large segment of the population in your community smokes. In order to convince your patient population that it would be beneficial to quit smoking in order to prevent lung cancer, you conduct a casecontrol study of cigarette smoking and lung cancer. The results from that study can be found in the table below.

  

  What is the odds ratio (OR) in your study?

  A. 3

  B. 6

  C. 9

  D. 12

  E. 20

  Correct Answer: C

  The OR is used when the denominator data are missing. It is calculated using the following formula,

  comparing smokers and nonsmokers:

  a x d / b x c

  In this case, the mathematics is relatively simple: 75 x 75/25 x 25. By reduction, this is 3 x 3/1 x 1.

• Question 379:

  The crude death rate in Sweden was 0.010 per year, while in Costa Rica it was 0.008 per year. All age-specific death rates, except those for the oldest-age category, were higher in Costa Rica than in Sweden. From these data, one can correctly infer which of the following?

  A. The difference is too small for any deductions to be made.

  B. It is healthier to live in Sweden than in Costa Rica.

  C. There is less cardiovascular disease in Costa Rica than in Sweden.

  D. A greater proportion of the Swedish population is in the older-age categories.

  E. There is unexplained progressive deterioration of health indicators in Costa Rica relative to those of Sweden.

  Correct Answer: D

  In a crude death rate, all deaths are in the numerator, and the total midyear population in the denominator.

  In age-specific death rates, the calculation is done using data from specific age intervals. Small differences in crude death rates may enable specific deductions to be made. Ahigher death rate in the older population may indicate that a more fragile population has successfully survived to that age. Of the statements listed, considering that age-specific death rates were greater in all age groups except the elderly, only the fact that a greater proportion of the population in Sweden is in the older age groups could account for the difference in crude mortality rate.

• Question 380:

  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. The variance of the set of values is 58.75.

  What is the standard deviation (SD)?

  A. 5.8

  B. 6.5

  C. 7.7

  D. 8.5

  E. 9.0

  Correct Answer: C

  The variance is the sum of the squares of the difference of each reading from the mean. This is a mathematical way of eliminating the opposing effects of negative and positive values. But the variance has a result that is of a different order than the original observations. The variance of 58.75 seems to have little relationship to a series of readings from 61 to 80. In order to determine a value that is the same order as the observations, the SD is calculated as the square root of the variance. This is mathematically represented as:

  

  Without using a calculator, it can be seen that the nearest correct answer (correct to 1 decimal place) is

  7.7.