1. Topic: Study Design

Correct Answer: C. A prospective cohort

Explanation: In a prospective cohort study, participants are followed over time to observe outcomes based on exposure status. In this case, women with a history of smoking are followed to determine their babies’ birth weights. This design allows for tracking the association between smoking and birth weight over time. Other designs like clinical trials (A), cross-sectional studies (B), case-control studies (D), and retrospective cohort studies (E) do not fit this description as closely as the prospective cohort design.

2. Topic: Blood Pressure Measurement and Study Design

Correct Answer: B. Regression to the mean

Explanation: Regression to the mean occurs when subjects with extreme measurements at the start of a study tend to have more average measurements upon re-evaluation, simply due to statistical variability. In this case, patients with high diastolic blood pressure may show a reduction over time, leading to fewer patients meeting the study’s criteria upon follow-up. Spontaneous resolution (A) and measurement error (D) are less likely to fully explain the reduction in qualifying patients, while basic drift (C) and the Hawthorne effect (E) do not directly address the statistical phenomenon observed.

3. Topic: Incidence Calculation

Correct Answer: D. Person-years of observation

Explanation: Person-years of observation is used as the denominator in calculating incidence rates to account for both the number of people and the time each person is at risk. This measure helps to reflect the time each individual contributes to the risk of developing the disease. The number of new cases observed (B) is the numerator for incidence calculation, while the number of cases observed (A) and asymptomatic cases (C) are not directly used. Persons lost to follow-up (E) do not contribute to the denominator but can affect the study’s accuracy.

4. Topic: Normal Distribution and Weight Analysis

Correct Answer: D. 16%

Explanation: To determine the percentage of women who weigh more than 59.5 kg, we need to find the z-score for 59.5 kg using the mean (52 kg) and standard deviation (7.5 kg).

The z-score is calculated as:

z=(X−mean)standard deviation=(59.5−52)7.5=1z = frac{(X – text{mean})}{text{standard deviation}} = frac{(59.5 – 52)}{7.5} = 1z=standard deviation(X−mean)​=7.5(59.5−52)​=1

A z-score of 1 corresponds to approximately the 84th percentile of the normal distribution, meaning that about 16% of women weigh more than 59.5 kg.

5. Topic: Measures of Central Tendency

Correct Answer: E. 3, 4, 3

Explanation: To find the mean, median, and mode:

1. Mean: Sum of all values divided by the number of values. Mean=4+6+2+2+4+3+2+1+79=309=3.33text{Mean} = frac{4 + 6 + 2 + 2 + 4 + 3 + 2 + 1 + 7}{9} = frac{30}{9} = 3.33Mean=94+6+2+2+4+3+2+1+7​=930​=3.33 Approximated to the nearest integer, the mean is 3.
2. Median: The middle value when the numbers are sorted in ascending order. Sorted numbers: 1, 2, 2, 3, 4, 4, 6, 7 The median is 4.
3. Mode: The value that appears most frequently. In this case, the mode is 3 (as 3 appears most frequently).

Thus, the mean is 3, the median is 4, and the mode is 3.

6. Topic: Analyzing Data for Association Studies

Correct Answer: C. Fisher’s exact test

Explanation: Fisher’s exact test is used to determine if there are nonrandom associations between two categorical variables, especially in cases where sample sizes are small. In this scenario, where the association between coffee consumption (more than eight cups vs. not drinking coffee) and hypertension is being assessed, Fisher’s exact test is appropriate for analyzing the categorical data from a 2×2 contingency table. The Chi-square test (A) is typically used for larger sample sizes. The McNemar test (B) is used for paired nominal data, which doesn’t apply here. Student’s t-test (D) is used for comparing means between two groups, and Analysis of Variance (E) is used for comparing means among more than two groups.

7. Topic: Incidence Rate Calculation

Correct Answer: D. 10

Explanation: To calculate the incidence rate per 1000 people per year in households with a culture-positive case at the first survey, follow these steps:

1. Determine the number of new cases in the households with a culture-positive case during the follow-up period.
2. Find the total number of people at risk in those households.
3. Calculate the incidence rate using the formula: Incidence rate=Number of new casesTotal person-years at risk×1000text{Incidence rate} = frac{text{Number of new cases}}{text{Total person-years at risk}} times 1000Incidence rate=Total person-years at riskNumber of new cases​×1000

If the study data results in 10 new cases per 1000 people per year, this is the incidence rate for the households with a culture-positive case from the first survey.

The other options (A, B, C, E) do not correctly reflect the given incidence rate calculation.

8. Topic: Incidence Rate Calculation in Households Without Culture-Positive Cases

Correct Answer: C. 0.5

Explanation: To calculate the incidence rate of new cases per 1000 person-years in households that had no culture-positive cases at the first survey:

1. Identify the number of new cases in these households during the follow-up period.
2. Determine the total person-years at risk for these households.
3. Apply the formula: Incidence rate=Number of new casesTotal person-years at risk×1000text{Incidence rate} = frac{text{Number of new cases}}{text{Total person-years at risk}} times 1000Incidence rate=Total person-years at riskNumber of new cases​×1000

If the incidence rate is calculated to be 0.5 per 1000 person-years, this reflects the rate of new cases in households without initial culture-positive cases.

Other options (A, B, D, E) do not accurately represent the incidence rate of 0.5 per 1000 person-years.

9. Topic: Relative Risk of Contracting TB

Correct Answer: E. 20

Explanation: To calculate the relative risk (RR) of contracting TB in households with culture-positive cases compared to those without:

1. Determine the incidence rate of TB in households with culture-positive cases.
2. Determine the incidence rate of TB in households without culture-positive cases.
3. Apply the formula: Relative Risk (RR)=Incidence rate in exposed groupIncidence rate in unexposed grouptext{Relative Risk (RR)} = frac{text{Incidence rate in exposed group}}{text{Incidence rate in unexposed group}}Relative Risk (RR)=Incidence rate in unexposed groupIncidence rate in exposed group​

Given that the relative risk is 20, this indicates that individuals in households with culture-positive cases are 20 times more likely to contract TB compared to those in households without initial culture-positive cases.

Other options (A, B, C, D) do not accurately reflect the significant increase in risk associated with culture-positive households.

10. Topic: Experimental vs. Observational Studies

Correct Answer: E. Investigators determine who will or will not be exposed to the suspected causative factor

Explanation: In an experimental study, such as a randomized controlled trial, the researchers actively intervene and assign participants to either the exposed or non-exposed group to study the effect of an intervention or exposure. This control over exposure is what distinguishes experimental studies from observational studies. In observational studies, researchers simply observe and record exposures and outcomes without assigning exposure.

11. Topic: Rabies Prophylaxis in Children

Correct Answer: C. Administer rabies vaccine and rabies immunoglobulin

Explanation: Given the history of potential exposure to a bat, which is a known rabies vector, and the absence of visible scratches, it is still prudent to administer both rabies vaccine and rabies immunoglobulin. Rabies is a serious disease, and prophylactic treatment is essential even if no physical evidence of a bite or scratch is present. The combination of vaccine and immunoglobulin provides comprehensive protection against rabies.

12. Topic: Vaccination During Pregnancy

Correct Answer: B. Varicella vaccine

Explanation: The varicella (chickenpox) vaccine is a live attenuated vaccine and is contraindicated during pregnancy due to the potential risk to the fetus. Pregnant women should avoid this vaccine, as live vaccines can pose a risk to the developing baby.

13. Topic: Tetanus Prophylaxis

Correct Answer: A. No additional prophylaxis

Explanation: Given that the patient received the last dose of tetanus toxoid 7 years ago and has completed a primary series of immunizations, no additional prophylaxis is needed for this minor injury. Tetanus prophylaxis recommendations generally suggest a booster every 10 years, but since the injury is not classified as high risk and the patient is up-to-date on vaccinations, additional prophylaxis is not required.

14. Topic: Interval Between Whole Blood Administration and MMR Vaccine

Correct Answer: D. 6

Explanation: It is recommended to wait at least 6 months after receiving whole blood before administering the measles-mumps-rubella (MMR) vaccine. This is because whole blood transfusions can interfere with the effectiveness of live vaccines like MMR. Waiting 6 months ensures that the vaccine will be effective.

15. Topic: Etiological Agent of Lower Abdominal Pain and Cervical Findings

Correct Answer: B. Chlamydia trachomatis

Explanation: Chlamydia trachomatis is a common cause of cervicitis and can present with symptoms such as lower abdominal pain, friable cervix, and pain on cervical movement. This sexually transmitted infection often causes these findings without fever and can be associated with negative pregnancy tests.

16. Topic: Risk of Infertility from Pelvic Inflammatory Disease (PID)

Correct Answer: D. 20%

Explanation: Pelvic inflammatory disease (PID) can increase the risk of infertility due to scarring and damage to the reproductive organs. Each episode of PID contributes to this risk, and after two episodes, the cumulative risk of infertility is around 20%. This risk reflects the potential for complications such as blocked fallopian tubes and other reproductive tract issues.

17. Topic: Diagnosis of Genital Ulcers

Correct Answer: A. Syphilis

Explanation: A single, hard, painless ulcer on the penis that has been present for 2 days and is consistent with a primary chancre is characteristic of primary syphilis. The RPR test may be negative in the early stages of syphilis, as it can take several weeks for the antibodies to become detectable.

18. Topic: Sexual Partner Notification

Correct Answer: B. Partners less than 30 days old

Explanation: For sexually transmitted infections (STIs) like syphilis, partners from the last 30 days should be informed and evaluated. This timeframe is based on the typical incubation period of STIs, ensuring that recent exposures are addressed effectively to prevent further transmission and complications.

19. Topic: MMR Vaccination Safety

Correct Answer: A. A 15-month-old HIV infected child with a CD4 count of 700

Explanation: MMR (measles, mumps, rubella) vaccine is live attenuated and can be safely administered to children with HIV if their CD4 count is above 500 cells/mm³, indicating good immune function. A CD4 count of 700 suggests adequate immune status for receiving live vaccines.

20. Topic: HCV-Positive Women Counseling

Correct Answer: B. The probability of transmission to the newborn is 5%

Explanation: Hepatitis C virus (HCV) transmission from mother to infant occurs in about 5% of cases, which is a key counseling point for HCV-positive women. This transmission rate helps in understanding the risk and planning appropriate follow-up for the newborn.