Principles of screening US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Principles of screening. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Principles of screening US Medical PG Question 1: A 36-year-old female presents to clinic inquiring about the meaning of a previous negative test result from a new HIV screening test. The efficacy of this new screening test for HIV has been assessed by comparison against existing gold standard detection of HIV RNA via PCR. The study includes 1000 patients, with 850 HIV-negative patients (by PCR) receiving a negative test result, 30 HIV-negative patients receiving a positive test result, 100 HIV positive patients receiving a positive test result, and 20 HIV positive patients receiving a negative test result. Which of the following is most likely to increase the negative predictive value for this test?
- A. Decreased prevalence of HIV in the tested population (Correct Answer)
- B. Increased prevalence of HIV in the tested population
- C. Increased number of false positive test results
- D. Increased number of false negative test results
- E. Decreased number of false positive test results
Principles of screening Explanation: ***Decreased prevalence of HIV in the tested population***
- A **lower prevalence** of a disease in the population means there are fewer actual cases, making a **negative test result** more reliable in ruling out the disease.
- This increases the probability that a person with a negative test truly does not have the disease, thus elevating the **negative predictive value (NPV)**.
*Increased prevalence of HIV in the tested population*
- A **higher prevalence** means there are more actual cases of HIV in the population.
- In this scenario, a negative test result is less reassuring, as there's a greater chance of missing a true positive case, leading to a **decreased NPV**.
*Increased number of false positive test results*
- **False positives** are instances where a test indicates disease when it's not present; they do not directly impact the ability of a negative test to predict absence of disease.
- While they affect the **positive predictive value (PPV)**, they do not directly alter the reliability of a negative result to exclude disease, so the NPV is not increased.
*Increased number of false negative test results*
- **False negatives** occur when a test indicates no disease, but the disease is actually present.
- An increase in false negatives directly implies that a negative test result is less trustworthy, leading to a **decrease in the NPV**.
*Decreased number of false positive test results*
- A decrease in false positive results primarily improves the **positive predictive value (PPV)**.
- While it indicates a more accurate test overall, it does not directly affect NPV, which measures the reliability of a negative test result in ruling out disease.
Principles of screening US Medical PG Question 2: A mother presents to the family physician with her 16-year-old son. She explains, "There's something wrong with him doc. His grades are getting worse, he's cutting class, he's gaining weight, and his eyes are often bloodshot." Upon interviewing the patient apart from his mother, he seems withdrawn and angry at times when probed about his social history. The patient denies abuse and sexual history. What initial test should be sent to rule out the most likely culprit of this patient's behavior?
- A. Complete blood count
- B. Sexually transmitted infection (STI) testing
- C. Blood culture
- D. Urine toxicology screen (Correct Answer)
- E. Slit lamp examination
Principles of screening Explanation: ***Urine toxicology screen***
- The patient's presentation with **declining grades**, **cutting class**, **weight gain**, **bloodshot eyes**, and **irritability** are classic signs of **substance abuse** in an adolescent.
- A **urine toxicology screen** is the most appropriate initial test to detect common illicit substances, especially given the clear signs pointing towards drug use.
*Slit lamp examination*
- This test is used to examine the **anterior segment of the eye**, including the conjunctiva, cornea, iris, and lens.
- While the patient has **bloodshot eyes**, this specific test would be more relevant for ruling out ocular infections or injuries, not for diagnosing the underlying cause of systemic behavioral changes.
*Complete blood count*
- A **complete blood count (CBC)** measures different components of the blood, such as red blood cells, white blood cells, and platelets.
- A CBC is a general health indicator and while it can detect infections or anemia, it is not specific or sensitive enough to identify the cause of the behavioral changes described.
*Sexually transmitted infection (STI) testing*
- Although the patient denies sexual history, all adolescents presenting with certain risk factors or symptoms may warrant STI testing in a broader health assessment.
- However, in this scenario, the primary cluster of symptoms (poor grades, cutting class, bloodshot eyes, irritability) points more directly to substance abuse than to an STI.
*Blood culture*
- A **blood culture** is used to detect the presence of bacteria or other microorganisms in the bloodstream, indicating a systemic infection (sepsis).
- The patient's symptoms are not indicative of an acute bacterial bloodstream infection, and a blood culture would not be the initial test for the presented behavioral changes.
Principles of screening US Medical PG Question 3: A home drug screening test kit is currently being developed. The cut-off level is initially set at 4 mg/uL, which is associated with a sensitivity of 92% and a specificity of 97%. How might the sensitivity and specificity of the test change if the cut-off level is changed to 2 mg/uL?
- A. Sensitivity = 92%, specificity = 97%
- B. Sensitivity = 95%, specificity = 98%
- C. Sensitivity = 100%, specificity = 97%
- D. Sensitivity = 90%, specificity = 99%
- E. Sensitivity = 97%, specificity = 96% (Correct Answer)
Principles of screening Explanation: ***Sensitivity = 97%, specificity = 96%***
- Lowering the cut-off from 4 mg/uL to 2 mg/uL means that more individuals will be classified as **positive** (anyone with drug levels ≥2 mg/uL instead of ≥4 mg/uL). This change will **increase the sensitivity** (capturing more true positives, fewer false negatives) but **decrease the specificity** (more false positives among those without the condition).
- Therefore, sensitivity will increase (e.g., to 97%), and specificity will decrease (e.g., to 96%), reflecting the fundamental trade-off between these metrics.
*Sensitivity = 92%, specificity = 97%*
- This option reflects the **original values** at the 4 mg/uL cut-off and does not account for the change in the threshold.
- A change in the cut-off level will inherently alter the test's performance characteristics.
*Sensitivity = 95%, specificity = 98%*
- This option suggests an increase in **both sensitivity and specificity**, which is generally not possible by simply changing the cut-off level in the same direction.
- There is typically an **inverse relationship** between sensitivity and specificity when adjusting the cut-off threshold.
*Sensitivity = 100%, specificity = 97%*
- Reaching **100% sensitivity** while maintaining a high specificity is highly unlikely with a simple cut-off adjustment.
- While sensitivity would increase with a lower cut-off, achieving perfect sensitivity is unrealistic in clinical practice.
*Sensitivity = 90%, specificity = 99%*
- This option suggests a **decrease in sensitivity** and an **increase in specificity**.
- A lower cut-off would lead to more positive results, thus increasing sensitivity and reducing specificity, which contradicts the proposed values.
Principles of screening US Medical PG Question 4: A genetic population study is being conducted to find the penetrance of a certain disease. This disease is associated with impaired iron metabolism and primarily affects the liver. Patients often present with diabetes and bronze skin pigmentation. After a genetic screening of 120 inhabitants with a family history of this disease, 40 were found to have the disease-producing genotype, but only 10 presented with symptoms. What are the chances of the screened patients with said genotype developing the disease phenotype?
- A. 0.4%
- B. 40%
- C. 3%
- D. 4%
- E. 25% (Correct Answer)
Principles of screening Explanation: ***25%***
- **Penetrance** is calculated as the proportion of individuals with a specific genotype who express the associated phenotype.
- In this case, 10 individuals out of 40 with the disease-producing genotype developed symptoms, so (10 / 40) * 100% = **25%**.
*0.4%*
- This value is significantly lower than the actual penetrance and likely results from an incorrect calculation or misinterpretation of the given data.
- It does not accurately reflect the proportion of genotypically affected individuals who express the phenotype.
*40%*
- This percentage represents the proportion of screened individuals with the disease-producing genotype (40 out of 120 are ~33%), not the penetrance itself.
- It incorrectly equates the presence of the genotype in the population with the expression of the phenotype.
*3%*
- This value is likely obtained by an erroneous calculation, possibly by dividing the symptomatic individuals by the total screened population (10/120 ≈ 8.3%), which does not represent penetrance.
- It does not account for the specific individuals who possess the genotype.
*4%*
- This percentage might arise from an incorrect division or a misunderstanding of what constitutes penetrance.
- It is an inaccurate representation of the ratio between phenotype expression and genotype presence.
Principles of screening US Medical PG Question 5: A 57-year-old man presents for his yearly wellness visit. He says he has been feeling well and has no complaints. No significant past medical history or current medications. The patient reports a 35-pack-year smoking history but says he quit 5 years ago. His family history is significant for lung cancer in his father who died at age 67. His vital signs include: temperature 36.8°C (98.2°F), pulse 95/min, respiratory rate 16/min, blood pressure 100/75 mm Hg. Physical examination is unremarkable. Laboratory findings are significant for the following:
Serum potassium 3.9 mEq/L
Serum sodium 140 mEq/L
Serum chloride 103 mEq/L
Serum calcium 2.5 mmol/L
BUN 15 mg/dL
Creatinine 0.8 mg/dL
Glucose 95 mg/dL
Magnesium 1.7 mEq/L
Phosphate 1.1 mmol/L
Hemoglobin 14 g/dL
Bicarbonate (HCO3-) 25 mEq/L
Bilirubin, total 0.9 mg/dL
Bilirubin, indirect 0.4 mg/dL
AST 10 U/L
ALT 19 U/L
Alkaline phosphatase 40 U/L
Albumin 3.6 g/dL
Which of the following preventative screening tests is recommended for this patient at this time?
- A. Abdominal ultrasound
- B. No screening indicated
- C. Low-dose CT scan (LDCT) of the chest (Correct Answer)
- D. ECG
- E. Chest X-ray
Principles of screening Explanation: ***Low-dose CT scan (LDCT) of the chest***
- The patient meets the criteria for **lung cancer screening** due to his age (57), significant **smoking history** (35 pack-years), and quitting within the last 15 years (5 years ago).
- Guidelines recommend annual **LDCT** for individuals aged 50-80 with a 20+ pack-year smoking history who currently smoke or have quit within the past 15 years.
*Abdominal ultrasound*
- This test is not routinely recommended for asymptomatic individuals for general screening; its use is typically for specific symptoms or risks (e.g., abdominal pain, family history of abdominal aortic aneurysm).
- The patient has no symptoms or risk factors that would suggest the need for an abdominal ultrasound at this time.
*No screening indicated*
- Given the patient's **heavy smoking history** and age, there is a clear indication for **lung cancer screening**.
- Skipping screening would miss an opportunity for early detection of lung cancer, which is crucial for improving outcomes.
*ECG*
- An **ECG** screens for cardiac abnormalities but is not a primary screening tool for lung cancer.
- While smoking is a risk factor for cardiovascular disease, the immediate and most pressing screening need based on his history is related to lung cancer.
*Chest X-ray*
- A **chest X-ray** is generally not recommended for **lung cancer screening** in high-risk individuals because it has lower sensitivity compared to **LDCT** for detecting early-stage tumors.
- **LDCT** is the preferred method for lung cancer screening due to its superior ability to detect small, actionable lesions.
Principles of screening US Medical PG Question 6: A 50-year-old woman comes to the physician for a routine health maintenance examination. She has no personal or family history of serious illness. She smoked one pack of cigarettes daily for 5 years during her 20s. Her pulse is 70/min, and blood pressure is 120/78 mm Hg. Serum lipid studies and glucose concentration are within the reference ranges. Which of the following health maintenance recommendations is most appropriate at this time?
- A. Perform BRCA gene test
- B. Perform abdominal ultrasound
- C. Perform 24-hour ECG
- D. Perform DEXA scan
- E. Perform colonoscopy (Correct Answer)
Principles of screening Explanation: ***Perform colonoscopy***
- **Colorectal cancer screening** with colonoscopy is generally recommended for individuals at average risk starting at age **45-50 years**.
- This patient is **50 years old** and has no increased risk factors, making routine colonoscopy the most appropriate screening.
*Perform BRCA gene test*
- **BRCA gene testing** is indicated for individuals with a strong **family history** of breast or ovarian cancer, or those with personal history suggesting a genetic predisposition.
- This patient has **no personal or family history** of serious illness, so BRCA testing is not warranted.
*Perform abdominal ultrasound*
- **Abdominal ultrasound** is typically used to investigate specific symptoms or screen for conditions like **abdominal aortic aneurysm** in high-risk individuals (e.g., male smokers over 65).
- This patient has **no relevant symptoms** or risk factors for which routine abdominal ultrasound screening is recommended.
*Perform 24-hour ECG*
- A **24-hour ECG (Holter monitor)** is used to detect paroxysmal **arrhythmias** or evaluate symptoms like palpitations, syncope, or dizziness.
- This patient is asymptomatic with a normal pulse and blood pressure; therefore, **routine 24-hour ECG** is not indicated.
*Perform DEXA scan*
- A **DEXA scan** is recommended for **osteoporosis screening** in women typically starting at age **65**, or earlier if they have significant risk factors like a history of fragility fractures or certain medical conditions.
- At **50 years old** and with no apparent risk factors for osteoporosis, a DEXA scan is not yet routinely indicated according to general guidelines.
Principles of screening US Medical PG Question 7: A medical research study is beginning to evaluate the positive predictive value of a novel blood test for non-Hodgkin’s lymphoma. The diagnostic arm contains 700 patients with NHL, of which 400 tested positive for the novel blood test. In the control arm, 700 age-matched control patients are enrolled and 0 are found positive for the novel test. What is the PPV of this test?
- A. 400 / (400 + 0) (Correct Answer)
- B. 700 / (700 + 300)
- C. 400 / (400 + 300)
- D. 700 / (700 + 0)
- E. 700 / (400 + 400)
Principles of screening Explanation: ***400 / (400 + 0) = 1.0 or 100%***
- The **positive predictive value (PPV)** is calculated as **True Positives / (True Positives + False Positives)**.
- In this scenario, **True Positives (TP)** are the 400 patients with NHL who tested positive, and **False Positives (FP)** are 0, as no control patients tested positive.
- This gives a PPV of 400/400 = **1.0 or 100%**, indicating that all patients who tested positive actually had the disease.
*700 / (700 + 300)*
- This calculation does not align with the formula for PPV based on the given data.
- The denominator `(700+300)` suggests an incorrect combination of various patient groups.
*400 / (400 + 300)*
- The denominator `(400+300)` incorrectly includes 300, which is the number of **False Negatives** (patients with NHL who tested negative), not False Positives.
- PPV focuses on the proportion of true positives among all positive tests, not all diseased individuals.
*700 / (700 + 0)*
- This calculation incorrectly uses the total number of patients with NHL (700) as the numerator, rather than the number of positive test results in that group.
- The numerator should be the **True Positives** (400), not the total number of diseased individuals.
*700 / (400 + 400)*
- This calculation uses incorrect values for both the numerator and denominator, not corresponding to the PPV formula.
- The numerator 700 represents the total number of patients with the disease, not those who tested positive, and the denominator incorrectly sums up values that don't represent the proper PPV calculation.
Principles of screening US Medical PG Question 8: A student health coordinator plans on leading a campus-wide HIV screening program that will be free for the entire undergraduate student body. The goal is to capture as many correct HIV diagnoses as possible with the fewest false positives. The coordinator consults with the hospital to see which tests are available to use for this program. Test A has a sensitivity of 0.92 and a specificity of 0.99. Test B has a sensitivity of 0.95 and a specificity of 0.96. Test C has a sensitivity of 0.98 and a specificity of 0.93. Which of the following testing schemes should the coordinator pursue?
- A. Test A on the entire student body followed by Test B on those who are positive
- B. Test A on the entire student body followed by Test C on those who are positive
- C. Test C on the entire student body followed by Test B on those who are positive
- D. Test C on the entire student body followed by Test A on those who are positive (Correct Answer)
- E. Test B on the entire student body followed by Test A on those who are positive
Principles of screening Explanation: ***Test C on the entire student body followed by Test A on those who are positive***
- To "capture as many correct HIV diagnoses as possible" (maximize true positives), the initial screening test should have the **highest sensitivity**. Test C has the highest sensitivity (0.98).
- To "capture as few false positives as possible" (maximize true negatives and confirm diagnoses), the confirmatory test should have the **highest specificity**. Test A has the highest specificity (0.99).
*Test A on the entire student body followed by Test B on those who are positive*
- Starting with Test A (sensitivity 0.92) would miss more true positive cases than starting with Test C (sensitivity 0.98), failing the goal of **capturing as many cases as possible**.
- Following with Test B (specificity 0.96) would result in more false positives than following with Test A (specificity 0.99).
*Test A on the entire student body followed by Test C on those who are positive*
- This scheme would miss many true positive cases initially due to Test A's lower sensitivity compared to Test C.
- Following with Test C would introduce more false positives than necessary, as it has a lower specificity (0.93) than Test A (0.99).
*Test C on the entire student body followed by Test B on those who are positive*
- While Test C is a good initial screen for its high sensitivity, following it with Test B (specificity 0.96) is less optimal than Test A (specificity 0.99) for minimizing false positives in the confirmation step.
- This combination would therefore yield more false positives in the confirmatory stage than using Test A.
*Test B on the entire student body followed by Test A on those who are positive*
- Test B has a sensitivity of 0.95, which is lower than Test C's sensitivity of 0.98, meaning it would miss more true positive cases at the initial screening stage.
- While Test A provides excellent specificity for confirmation, the initial screening step is suboptimal for the goal of capturing as many diagnoses as possible.
Principles of screening US Medical PG Question 9: You are reviewing raw data from a research study performed at your medical center examining the effectiveness of a novel AIDS screening examination. The study enrolled 250 patients with confirmed AIDS, and 240 of these patients demonstrated a positive screening examination. The control arm of the study enrolled 250 patients who do not have AIDS, and only 5 of these patients tested positive on the novel screening examination. What is the NPV of this novel test?
- A. 240 / (240 + 15)
- B. 240 / (240 + 5)
- C. 240 / (240 + 10)
- D. 245 / (245 + 10) (Correct Answer)
- E. 245 / (245 + 5)
Principles of screening Explanation: ***245 / (245 + 10)***
- The **negative predictive value (NPV)** is calculated as **true negatives (TN)** divided by the sum of **true negatives (TN)** and **false negatives (FN)**.
- In this study, there are 250 patients with AIDS; 240 tested positive (true positives, TP), meaning 10 tested negative (false negatives, FN = 250 - 240). There are 250 patients without AIDS; 5 tested positive (false positives, FP), meaning 245 tested negative (true negatives, TN = 250 - 5). Therefore, NPV = 245 / (245 + 10).
*240 / (240 + 15)*
- This calculation incorrectly uses the number of **true positives** (240) in the numerator and denominator, which is relevant for **positive predictive value (PPV)**, not NPV.
- The denominator `(240 + 15)` does not correspond to a valid sum for calculating NPV from the given data.
*240 / (240 + 5)*
- This calculation incorrectly uses **true positives** (240) in the numerator, which is not part of the NPV formula.
- The denominator `(240 + 5)` mixes true positives and false positives, which is incorrect for NPV.
*240 / (240 + 10)*
- This incorrectly places **true positives** (240) in the numerator instead of **true negatives**.
- The denominator `(240+10)` represents **true positives + false negatives**, which is related to sensitivity, not NPV.
*245 / (245 + 5)*
- This calculation correctly identifies **true negatives** (245) in the numerator but incorrectly uses **false positives** (5) in the denominator instead of **false negatives**.
- The denominator for NPV should be **true negatives + false negatives**, which is 245 + 10.
Principles of screening US Medical PG Question 10: A 15-year-old Caucasian female presents with Parkinson-like symptoms. Serum analysis shows increased levels of free copper and elevated liver enzymes. What test would prove most helpful in diagnosing the patient's underlying disease?
- A. Slit lamp examination (Correct Answer)
- B. Reflex test
- C. Serum detection of anti-myelin antibodies
- D. Vitamin B12 test
- E. CT scan
Principles of screening Explanation: ***Slit lamp examination***
- The constellation of **Parkinson-like symptoms**, **elevated free copper** and **liver enzymes** in a young patient strongly suggests **Wilson's disease**.
- A slit lamp examination is crucial to detect **Kayser-Fleischer rings**, which are deposits of copper in the cornea and are pathognomonic for Wilson's disease.
*Reflex test*
- A reflex test assesses the integrity of the **peripheral nervous system** and **spinal cord reflexes**.
- While neurological symptoms are present, a reflex test would not directly help diagnose the underlying metabolic disorder of copper accumulation.
*Serum detection of anti-myelin antibodies*
- Detecting anti-myelin antibodies is relevant for demyelinating diseases like **multiple sclerosis**.
- This test is not indicated for a suspected **copper metabolism disorder** and would not explain the high free copper and liver enzyme levels.
*Vitamin B12 test*
- A Vitamin B12 test is used to diagnose **B12 deficiency**, which can cause neurological symptoms.
- However, B12 deficiency does not lead to **elevated free copper** or **liver enzyme abnormalities**.
*CT scan*
- A CT scan of the brain could reveal **basal ganglia abnormalities** often seen in Wilson's disease.
- However, a **slit lamp examination** for Kayser-Fleischer rings is a more specific and diagnostic test for Wilson's disease in this clinical context.
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