A 60-year-old man has baseline 10-year MI risk 30%; statin lowers it to 24% (ARR 6%, RRR 20%). Which statement about the number needed to treat (NNT) is correct?
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USMLE Step 1 Quiz
Practice Risk Measures And Screening in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 60-year-old man has baseline 10-year MI risk 30%; statin lowers it to 24% (ARR 6%, RRR 20%). Which statement about the number needed to treat (NNT) is correct?
This quiz focuses on Risk Measures And Screening, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.
A 60-year-old man has baseline 10-year MI risk 30%; statin lowers it to 24% (ARR 6%, RRR 20%). Which statement about the number needed to treat (NNT) is correct?
Explanation: This question tests understanding of risk measures and screening (USMLE Step 1). Risk measures like absolute risk reduction (ARR), relative risk reduction (RRR), and number needed to treat (NNT) help quantify treatment benefits and inform clinical decisions. In this vignette, NNT is key to determining how many patients need statin therapy to prevent one myocardial infarction over 10 years. Choice B is correct because NNT is calculated as 1/ARR, which is 1/0.06 ≈ 17. Choice A is incorrect due to mistakenly using an incorrect ARR value. Teaching strategies include memorizing the formulas for ARR, RRR, and NNT, and practicing their application in patient scenarios to appreciate how baseline risk influences treatment impact. Additionally, discuss these measures in shared decision-making to balance benefits and potential side effects.
A 50-year-old woman has mammography screening (sensitivity 90%, specificity 91%). Which of the following reflects the test's sensitivity?
Explanation: This question tests understanding of risk measures and screening (USMLE Step 1). Risk measures like sensitivity and specificity help understand test accuracy and implications on patient care. In this vignette, sensitivity is key to determining the test's ability to identify disease presence accurately. Choice A is correct because it reflects the test's sensitivity, defined as true positives among all with the disease. Choice B is incorrect due to misrepresenting sensitivity, as it would imply false negatives among diseased. Teaching strategies include focusing on understanding test characteristics like sensitivity and specificity, and applying them to patient care scenarios. Use 2x2 tables to calculate and visualize these measures in different prevalence settings.
A 60-year-old man begins BP therapy. Baseline 10-year stroke risk is 10%; treated risk is 8% (ARR 2%). Which statement about the number needed to treat (NNT) is correct?
Explanation: This question tests understanding of risk measures and screening (USMLE Step 1). Risk measures like absolute risk reduction (ARR), relative risk reduction (RRR), and number needed to treat (NNT) help quantify treatment benefits and inform clinical decisions. In this vignette, NNT is key to determining how many patients need blood pressure therapy to prevent one stroke over 10 years. Choice A is correct because NNT is calculated as 1/ARR, which is 1/0.02 = 50. Choice B is incorrect due to mistakenly using RRR or another value instead of ARR. Teaching strategies include memorizing the formulas for ARR, RRR, and NNT, and practicing their application in patient scenarios to appreciate how baseline risk influences treatment impact. Additionally, discuss these measures in shared decision-making to balance benefits and potential side effects.
A 60-year-old man starts statins. Trial data: MI risk 6% with placebo vs 4% with statin over 10 years (ARR 2%, RRR 33%). Which statement about the number needed to treat (NNT) is correct?
Explanation: This question tests understanding of risk measures and screening (USMLE Step 1). Risk measures like absolute risk reduction (ARR), relative risk reduction (RRR), and number needed to treat (NNT) help quantify treatment benefits and inform clinical decisions. In this vignette, NNT is key to determining how many patients need statin therapy to prevent one myocardial infarction over 10 years. Choice A is correct because NNT is calculated as 1/ARR, which is 1/0.02 = 50. Choice B is incorrect due to mistakenly using RRR (0.33) instead of ARR in the NNT formula. Teaching strategies include memorizing the formulas for ARR, RRR, and NNT, and practicing their application in patient scenarios to appreciate how baseline risk influences treatment impact. Additionally, discuss these measures in shared decision-making to balance benefits and potential side effects.
A 60-year-old man considers statins. Without treatment, 10-year MI risk is 20%; with statins it is 15% (ARR 5%, RRR 25%). Which statement about the number needed to treat (NNT) is correct?
Explanation: This question tests understanding of risk measures and screening (USMLE Step 1). Risk measures like absolute risk reduction (ARR), relative risk reduction (RRR), and number needed to treat (NNT) help quantify treatment benefits and inform clinical decisions. In this vignette, NNT is key to determining how many patients need statin therapy to prevent one myocardial infarction over 10 years. Choice B is correct because NNT is calculated as 1/ARR, which is 1/0.05 = 20. Choice A is incorrect due to mistakenly using RRR (0.25) instead of ARR in the NNT formula. Teaching strategies include memorizing the formulas for ARR, RRR, and NNT, and practicing their application in patient scenarios to appreciate how baseline risk influences treatment impact. Additionally, discuss these measures in shared decision-making to balance benefits and potential side effects.
A prospective cohort study follows 2000 individuals for 10 years to investigate the association between heavy alcohol consumption and the development of liver cirrhosis. Of the 1000 heavy drinkers, 80 develop cirrhosis. Of the 1000 non-drinkers, 10 develop cirrhosis.
Which of the following is the best estimate of the relative risk of developing cirrhosis associated with heavy alcohol consumption in this study?
Explanation: Relative risk (RR) is the ratio of the incidence of a disease in the exposed group to the incidence in the unexposed group. The incidence in the exposed group (heavy drinkers) is 80/1000 = 0.08. The incidence in the unexposed group (non-drinkers) is 10/1000 = 0.01. The relative risk is calculated as (Incidence in exposed) / (Incidence in unexposed) = 0.08 / 0.01 = 8.0.
A new screening test for an aggressive form of cancer is introduced. The test can detect the cancer 18 months earlier than the onset of clinical symptoms. After the test is widely adopted, epidemiologists note that the 5-year survival rate after diagnosis has significantly increased. However, the overall mortality rate from this cancer in the population has not changed.
Which of the following biases best explains the apparent increase in 5-year survival?
Explanation: Lead-time bias occurs when a screening test detects a disease earlier than it would have been detected by clinical symptoms. This increases the measured survival time from diagnosis, even if the treatment does not prolong life. The fact that the overall mortality rate is unchanged suggests that the early diagnosis did not alter the ultimate outcome, which is the hallmark of lead-time bias.
A randomized controlled trial is conducted to evaluate the efficacy of a new drug for preventing migraines. Over a one-year period, 15% of patients in the placebo group experienced a severe migraine, while 10% of patients in the treatment group experienced one.
What is the number needed to treat (NNT) to prevent one severe migraine over one year with this new drug?
Explanation: The number needed to treat (NNT) is the reciprocal of the absolute risk reduction (ARR). First, calculate the ARR: ARR = (Risk in control group) - (Risk in treatment group) = 15% - 10% = 5% or 0.05. Then, calculate the NNT: NNT = 1 / ARR = 1 / 0.05 = 20. This means 20 patients need to be treated for one year to prevent one severe migraine.
In a 5-year clinical trial for a new antihypertensive medication, 2000 patients with hypertension were randomized. In the control group of 1000 patients, 50 had a stroke. In the treatment group of 1000 patients, 30 had a stroke.
What is the absolute risk reduction (ARR) for stroke with the new medication?
Explanation: Absolute risk reduction (ARR) is the difference in the event rate between the control group and the treatment group. The event rate in the control group is 50/1000 = 5%. The event rate in the treatment group is 30/1000 = 3%. ARR = (Risk in control) - (Risk in treatment) = 5% - 3% = 2%.
A study is conducted on the risk of developing bladder cancer among 2000 chemical plant workers with known exposure to an industrial dye and 2000 office workers with no exposure. Over 20 years, 60 of the chemical plant workers develop bladder cancer, while 10 of the office workers develop bladder cancer.
What is the excess risk of bladder cancer per 1000 persons that is attributable to the chemical exposure?
Explanation: Attributable risk (also known as absolute risk increase or excess risk) is the difference in risk between the exposed and unexposed groups. Risk in exposed = 60/2000 = 0.03 or 30 per 1000. Risk in unexposed = 10/2000 = 0.005 or 5 per 1000. Attributable risk = 30 per 1000 - 5 per 1000 = 25 per 1000 persons.
A physician is considering two different medications for preventing cardiovascular events in high-risk patients. A meta-analysis shows that Drug A has a number needed to treat (NNT) of 25 over 5 years. A separate meta-analysis shows Drug B has an NNT of 75 for the same outcome over the same period.
Based on this information, which of the following is the most accurate conclusion?
Explanation: The number needed to treat (NNT) represents the number of patients who need to receive an intervention to prevent one additional adverse outcome. A lower NNT indicates a more effective intervention. Therefore, Drug A (NNT=25) is more effective than Drug B (NNT=75), as fewer patients need to be treated with Drug A to achieve the same benefit.
In a clinical trial for a new lipid-lowering agent, the 5-year risk of a major adverse cardiac event (MACE) was 4% in the treatment group and 6% in the placebo group.
What is the number of patients that must be treated for 5 years to prevent one MACE?
Explanation: The number needed to treat (NNT) is the reciprocal of the absolute risk reduction (ARR). First, calculate the ARR. ARR = Risk(placebo) - Risk(treatment) = 6% - 4% = 2% = 0.02. Next, calculate the NNT. NNT = 1 / ARR = 1 / 0.02 = 50. Therefore, 50 patients need to be treated for 5 years to prevent one major adverse cardiac event.
A new, highly sensitive imaging technique is developed that can detect very small, asymptomatic neuroendocrine tumors. After its introduction, the incidence of this type of tumor diagnosis doubles. However, long-term data show that the mortality rate from this cancer has not changed.
Which of the following concepts best explains the increased incidence without a change in mortality?
Explanation: Overdiagnosis is the detection of 'disease' that would never have caused symptoms or death. The highly sensitive test is likely identifying many indolent or non-progressive tumors that are clinically insignificant. This increases the number of diagnosed cases (incidence) but does not impact the mortality rate because these tumors were not destined to be lethal.
A physician reads a study reporting that the number needed to treat (NNT) with a certain statin to prevent one non-fatal myocardial infarction over 10 years is 30.
This NNT value is the reciprocal of which of the following?
Explanation: The number needed to treat (NNT) is a measure of the effectiveness of an intervention. It is mathematically defined as the inverse of the absolute risk reduction (ARR). The formula is NNT = 1 / ARR. Therefore, if the NNT is 30, the ARR is 1/30 or approximately 3.3%.
A new drug is tested in a high-risk population for preventing a specific adverse outcome. The study finds the number needed to treat (NNT) to prevent one event is 20. The drug is now being considered for use in a low-risk population where the baseline incidence of the adverse outcome is five times lower.
Assuming the relative risk reduction remains the same, how would the NNT be expected to change in the low-risk population?
Explanation: NNT is inversely proportional to the absolute risk reduction (ARR). ARR is calculated as (Baseline Risk) x (Relative Risk Reduction). If the RRR is constant but the baseline risk is 5 times lower in the new population, the ARR will also be 5 times lower. Since NNT = 1/ARR, a 5-fold decrease in ARR will lead to a 5-fold increase in NNT. Therefore, the new NNT would be 20 x 5 = 100.
A 10-year prospective study follows two groups of men aged 50-60. In the group with elevated serum cholesterol, the incidence of coronary artery disease (CAD) was 15%. In the group with normal serum cholesterol, the incidence of CAD was 3%.
The value 12% (0.12) represents which of the following measures?
Explanation: Attributable risk, also known as risk difference or absolute risk increase, is the difference in the rate of a condition between an exposed population and an unexposed population. It represents the excess risk due to the exposure. Here, it is calculated as (Incidence in exposed) - (Incidence in unexposed) = 15% - 3% = 12%.
Health officials are considering a population-wide screening program for a very rare but fatal disease. The screening test is highly sensitive (99%) but has a specificity of 90%.
What is the most significant problem with implementing this screening program in the general population?
Explanation: When a disease is rare, even a test with high specificity will produce a large number of false positives relative to true positives. This results in a low positive predictive value (PPV), which is the probability that a person with a positive test result truly has the disease. A low PPV means that most positive results will be false, leading to unnecessary anxiety, cost, and invasive follow-up procedures for many healthy individuals.
A clinical trial shows that a new intervention for smoking cessation increases the quit rate from 5% (placebo) to 15% (intervention) at one year.
Which of the following statements is the most accurate summary of these results?
Explanation: The number needed to treat (NNT) is the reciprocal of the absolute risk reduction (or absolute benefit increase). The absolute benefit is the difference in quit rates: 15% - 5% = 10% or 0.10. The NNT = 1 / 0.10 = 10. This means that for every 10 people who receive the intervention, one additional person will quit smoking compared to placebo. The other options are incorrect: the relative risk of remaining a smoker is (1-0.15)/(1-0.05) = 0.85/0.95 = 0.89 (not 0.85); the relative benefit is (15%-5%)/5% = 200%; and the intervention increases the quit rate by a factor of 3 (15%/5%), not 2.
A randomized controlled trial investigates a new oral anticoagulant for the prevention of deep vein thrombosis (DVT) in patients undergoing hip replacement surgery. In the control group, the incidence of DVT was 10%. In the treatment group, the incidence was 6%.
What is the relative risk reduction (RRR) of DVT with the new anticoagulant?
Explanation: Relative risk reduction (RRR) is the proportion of risk reduction attributable to the intervention compared to a control. RRR = (Risk in control - Risk in treatment) / Risk in control. ARR = 10% - 6% = 4%. RRR = 4% / 10% = 0.40, or 40%. Alternatively, Relative Risk (RR) = 6%/10% = 0.6. RRR = 1 - RR = 1 - 0.6 = 0.4 or 40%.
A large cohort study finds that individuals who regularly consume more than two servings of sugar-sweetened beverages per day have a relative risk of 1.8 for developing type 2 diabetes compared to those who consume none.
Which of the following is the most accurate interpretation of this finding?
Explanation: A relative risk (RR) of 1.8 means that the exposed group (consumers of sugar-sweetened beverages) is 1.8 times as likely to develop the outcome (diabetes) as the unexposed group. This can be expressed as an 80% increase in risk (RR of 1.0 means no change in risk; 1.8 - 1.0 = 0.8, or an 80% increase).