Bayesian approach to diagnosis US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Bayesian approach to diagnosis. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Bayesian approach to diagnosis US Medical PG Question 1: A scientist in Chicago is studying a new blood test to detect Ab to EBV with increased sensitivity and specificity. So far, her best attempt at creating such an exam reached 82% sensitivity and 88% specificity. She is hoping to increase these numbers by at least 2 percent for each value. After several years of work, she believes that she has actually managed to reach a sensitivity and specificity much greater than what she had originally hoped for. She travels to China to begin testing her newest blood test. She finds 2,000 patients who are willing to participate in her study. Of the 2,000 patients, 1,200 of them are known to be infected with EBV. The scientist tests these 1,200 patients' blood and finds that only 120 of them tested negative with her new exam. Of the patients who are known to be EBV-free, only 20 of them tested positive. Given these results, which of the following correlates with the exam's specificity?
- A. 82%
- B. 90%
- C. 84%
- D. 86%
- E. 98% (Correct Answer)
Bayesian approach to diagnosis Explanation: ***98%***
- **Specificity** measures the proportion of **true negatives** among all actual negatives.
- In this case, 800 patients are known to be EBV-free (actual negatives), and 20 of them tested positive (false positives). This means 800 - 20 = 780 tested negative (true negatives). Specificity = (780 / 800) * 100% = **98%**.
*82%*
- This value represents the *original sensitivity* before the scientist’s new attempts to improve the test.
- It does not reflect the *newly calculated specificity* based on the provided data.
*90%*
- This value represents the *newly calculated sensitivity* of the test, not the specificity.
- Out of 1200 EBV-infected patients, 120 tested negative (false negatives), meaning 1080 tested positive (true positives). Sensitivity = (1080 / 1200) * 100% = 90%.
*84%*
- This percentage is not directly derived from the information given for either sensitivity or specificity after the new test results.
- It does not correspond to any of the calculated values for the new test's performance.
*86%*
- This percentage is not directly derived from the information given for either sensitivity or specificity after the new test results.
- It does not correspond to any of the calculated values for the new test's performance.
Bayesian approach to diagnosis US Medical PG Question 2: A researcher is trying to determine whether a newly discovered substance X can be useful in promoting wound healing after surgery. She conducts this study by enrolling the next 100 patients that will be undergoing this surgery and separating them into 2 groups. She decides which patient will be in which group by using a random number generator. Subsequently, she prepares 1 set of syringes with the novel substance X and 1 set of syringes with a saline control. Both of these sets of syringes are unlabeled and the substances inside cannot be distinguished. She gives the surgeon performing the surgery 1 of the syringes and does not inform him nor the patient which syringe was used. After the study is complete, she analyzes all the data that was collected and performs statistical analysis. This study most likely provides which level of evidence for use of substance X?
- A. Level 3
- B. Level 1 (Correct Answer)
- C. Level 4
- D. Level 5
- E. Level 2
Bayesian approach to diagnosis Explanation: ***Level 1***
- The study design described is a **randomized controlled trial (RCT)**, which is considered the **highest level of evidence (Level 1)** in the hierarchy of medical evidence.
- Key features like **randomization**, **control group**, and **blinding (double-blind)** help minimize bias and strengthen the validity of the findings.
*Level 2*
- Level 2 evidence typically comprises **well-designed controlled trials without randomization** (non-randomized controlled trials) or **high-quality cohort studies**.
- While strong, they do not possess the same level of internal validity as randomized controlled trials.
*Level 3*
- Level 3 evidence typically includes **case-control studies** or **cohort studies**, which are observational designs and carry a higher risk of bias compared to RCTs.
- These studies generally do not involve randomization or intervention assignment by the researchers.
*Level 4*
- Level 4 evidence is usually derived from **case series** or **poor quality cohort and case-control studies**.
- These studies provide descriptive information or investigate associations without strong control for confounding factors.
*Level 5*
- Level 5 evidence is the **lowest level of evidence**, consisting of **expert opinion** or **animal research/bench research**.
- This level lacks human clinical data or systematic investigative rigor needed for higher evidence levels.
Bayesian approach to diagnosis US Medical PG Question 3: You are reading through a recent article that reports significant decreases in all-cause mortality for patients with malignant melanoma following treatment with a novel biological infusion. Which of the following choices refers to the probability that a study will find a statistically significant difference when one truly does exist?
- A. Type II error
- B. Type I error
- C. Confidence interval
- D. p-value
- E. Power (Correct Answer)
Bayesian approach to diagnosis Explanation: ***Power***
- **Power** is the probability that a study will correctly reject the null hypothesis when it is, in fact, false (i.e., will find a statistically significant difference when one truly exists).
- A study with high power minimizes the risk of a **Type II error** (failing to detect a real effect).
*Type II error*
- A **Type II error** (or **beta error**) occurs when a study fails to reject a false null hypothesis, meaning it concludes there is no significant difference when one actually exists.
- This is the **opposite** of what the question describes, which asks for the probability of *finding* a difference.
*Type I error*
- A **Type I error** (or **alpha error**) occurs when a study incorrectly rejects a true null hypothesis, concluding there is a significant difference when one does not actually exist.
- This relates to the **p-value** and the level of statistical significance (e.g., p < 0.05).
*Confidence interval*
- A **confidence interval** provides a range of values within which the true population parameter is likely to lie with a certain degree of confidence (e.g., 95%).
- It does not directly represent the probability of finding a statistically significant difference when one truly exists.
*p-value*
- The **p-value** is the probability of observing data as extreme as, or more extreme than, that obtained in the study, assuming the null hypothesis is true.
- It is used to determine statistical significance, but it is not the probability of detecting a true effect.
Bayesian approach to diagnosis US Medical PG Question 4: A pharmaceutical corporation is developing a research study to evaluate a novel blood test to screen for breast cancer. They enrolled 800 patients in the study, half of which have breast cancer. The remaining enrolled patients are age-matched controls who do not have the disease. Of those in the diseased arm, 330 are found positive for the test. Of the patients in the control arm, only 30 are found positive. What is this test’s sensitivity?
- A. 330 / (330 + 30)
- B. 330 / (330 + 70) (Correct Answer)
- C. 370 / (30 + 370)
- D. 370 / (70 + 370)
- E. 330 / (400 + 400)
Bayesian approach to diagnosis Explanation: ***330 / (330 + 70)***
- **Sensitivity** measures the proportion of actual **positives** that are correctly identified as such.
- In this study, there are **400 diseased patients** (half of 800). Of these, 330 tested positive (true positives), meaning 70 tested negative (false negatives). So sensitivity is **330 / (330 + 70)**.
*330 / (330 + 30)*
- This calculation represents the **positive predictive value**, which is the probability that subjects with a positive screening test truly have the disease. It uses **true positives / (true positives + false positives)**.
- It does not correctly calculate **sensitivity**, which requires knowing the total number of diseased individuals.
*370 / (30 + 370)*
- This expression is attempting to calculate **specificity**, which is the proportion of actual negatives that are correctly identified. It would be **true negatives / (true negatives + false positives)**.
- However, the numbers used are incorrect for specificity in this context given the data provided.
*370 / (70 + 370)*
- This formula is an incorrect combination of values and does not represent any standard epidemiological measure like **sensitivity** or **specificity**.
- It is attempting to combine false negatives (70) and true negatives (370 from control arm) in a non-standard way.
*330 / (400 + 400)*
- This calculation attempts to divide true positives by the total study population (800 patients).
- This metric represents the **prevalence of true positives within the entire study cohort**, not the test's **sensitivity**.
Bayesian approach to diagnosis US Medical PG Question 5: In 2013 the national mean score on the USMLE Step 1 exam was 227 with a standard deviation of 22. Assuming that the scores for 15,000 people follow a normal distribution, approximately how many students scored above the mean but below 250?
- A. 5,100 (Correct Answer)
- B. 4,500
- C. 6,000
- D. 3,750
- E. 6,750
Bayesian approach to diagnosis Explanation: ***5,100***
- To solve this, first calculate the **z-score** for 250: (250 - 227) / 22 = 1.045.
- Using a **z-table**, the area under the curve from the mean (z=0) to z=1.045 is approximately 0.353. Multiplying this by 15,000 students gives approximately **5,295 students**, which is closest to 5,100.
*4,500*
- This answer would imply a smaller proportion of students between the mean and 250 (around 30%), which is lower than the calculated z-score of 1.045 suggests.
- It does not accurately reflect the area under the **normal distribution curve** for the given range.
*6,000*
- This option would mean that approximately 40% of students scored in this range, which would correspond to a z-score much higher than 1.045 or a different standard deviation.
- This calculation overestimates the number of students within the specified range.
*3,750*
- This value represents 25% of the total students (15,000 * 0.25), indicating that only a quarter of the distribution lies in this range.
- This significantly underestimates the proportion of students scoring between the mean and 250 for the given standard deviation.
*6,750*
- This option reflects approximately 45% of the total student population (15,000 * 0.45), which would correspond to a much larger z-score or a different distribution.
- This value is an overestimation and does not align with the standard normal distribution probabilities for the given parameters.
Bayesian approach to diagnosis US Medical PG Question 6: A 27-year-old woman presents to her primary care physician for minor aches and pains in her bones and muscles. She states that these symptoms have persisted throughout her entire life but have worsened recently when she moved to attend college. The patient is physically active, and states that she eats a balanced diet. She is currently a full-time student and is sexually active with 1 partner. She states that she has been particularly stressed lately studying for final exams and occasionally experiences diarrhea. She has been taking acyclovir for a dermatologic herpes simplex virus infection with minimal improvement. On physical exam, the patient exhibits 4/5 strength in her upper and lower extremities, and diffuse tenderness over her limbs that is non-specific. Laboratory values are ordered as seen below:
Serum:
Na+: 144 mEq/L
Cl-: 102 mEq/L
K+: 4.7 mEq/L
HCO3-: 24 mEq/L
Ca2+: 5.0
Urea nitrogen: 15 mg/dL
Glucose: 81 mg/dL
Creatinine: 1.0 mg/dL
Alkaline phosphatase: 225 U/L
Aspartate aminotransferase (AST, GOT): 11 U/L
Alanine aminotransferase (ALT, GPT): 15 U/L
Which of the following is most likely associated with this patient’s presentation?
- A. Vitamin D deficiency (Correct Answer)
- B. Fibromyalgia
- C. Primary hyperparathyroidism
- D. Chronic fatigue syndrome
- E. Systemic lupus erythematosus
Bayesian approach to diagnosis Explanation: ***Correct: Vitamin D deficiency***
- The patient presents with the **classic biochemical findings of vitamin D deficiency**: **severe hypocalcemia (Ca2+ 5.0 mg/dL, normal 8.5-10.5)** and **elevated alkaline phosphatase (225 U/L)**.
- **Clinical features of osteomalacia** are present: diffuse bone and muscle pain, proximal muscle weakness (4/5 strength in extremities), and bone tenderness—all consistent with bone demineralization and secondary myopathy.
- **Risk factor identified**: Recent move to college may represent lifestyle changes including reduced sun exposure, dietary changes, or increased indoor time studying.
- The elevated alkaline phosphatase reflects increased osteoblastic activity attempting to compensate for undermineralized bone matrix.
- Severe vitamin D deficiency also impairs immune function, which may explain the herpes simplex infection with poor response to acyclovir.
*Incorrect: Chronic fatigue syndrome*
- Chronic fatigue syndrome (CFS) is a **diagnosis of exclusion** characterized by persistent unexplained fatigue for at least 6 months with **normal laboratory findings**.
- This patient has **significant biochemical abnormalities** (severe hypocalcemia, elevated alkaline phosphatase) that exclude CFS and point to a specific metabolic disorder.
- CFS does not cause hypocalcemia, elevated alkaline phosphatase, or objective muscle weakness on examination.
*Incorrect: Fibromyalgia*
- While fibromyalgia presents with widespread musculoskeletal pain, it is characterized by **normal laboratory studies** including normal calcium and alkaline phosphatase.
- The patient's severe hypocalcemia and elevated alkaline phosphatase exclude fibromyalgia as the primary diagnosis.
- Fibromyalgia typically requires identification of specific tender points on examination, which are not described here.
*Incorrect: Systemic lupus erythematosus*
- SLE typically presents with **multisystem involvement** including malar rash, photosensitivity, serositis, nephritis, and hematologic abnormalities.
- Laboratory findings would show **positive autoantibodies** (ANA, anti-dsDNA, anti-Smith), not isolated hypocalcemia with elevated alkaline phosphatase.
- The patient's normal liver and kidney function, absence of systemic features, and specific biochemical pattern do not support SLE.
*Incorrect: Primary hyperparathyroidism*
- Primary hyperparathyroidism is defined by **hypercalcemia** with elevated or inappropriately normal PTH levels.
- This patient has **severe hypocalcemia (Ca2+ 5.0)**, which is the **opposite** of what occurs in hyperparathyroidism.
- The biochemical pattern (low calcium, high alkaline phosphatase) is consistent with hypoparathyroidism or vitamin D deficiency, not hyperparathyroidism.
Bayesian approach to diagnosis US Medical PG Question 7: A 63-year-old woman with a past medical history significant for hypertension presents to the outpatient clinic for evaluation of vaginal dryness, loss of libido, and hot flashes. These symptoms have been progressively worsening over the past 3 months. Her vital signs are: blood pressure 131/81 mm Hg, pulse 68/min, and respiratory rate 16/min. She is afebrile. On further review of systems, she endorses having irregular periods for almost a year, and asks if she has begun menopause. Which of the following parameters is required to formally diagnosis menopause in this patient?
- A. Increased serum follicle-stimulating hormone (FSH)
- B. Increased serum luteinizing hormone (LH)
- C. Cessation of menses for at least 12 months (Correct Answer)
- D. Pelvic ultrasound demonstrating decreased follicular activity
- E. Increased total cholesterol
Bayesian approach to diagnosis Explanation: ***Cessation of menses for at least 12 months***
- The formal diagnosis of **menopause** is clinical, defined as 12 consecutive months of **amenorrhea** in the absence of other physiological or pathological causes.
- This criterion indicates the **cessation of ovarian function** without requiring lab tests, especially in women over 40.
*Increased serum follicle-stimulating hormone (FSH)*
- While an **elevated FSH level** is a biochemical indicator of declining ovarian function, it is not strictly required for a clinical diagnosis of menopause, particularly in older women with typical symptoms and amenorrhea.
- FSH levels can fluctuate during the **perimenopausal period**, making a single measurement less definitive than the clinical criterion of 12 months without menses.
*Increased serum luteinizing hormone (LH)*
- Similar to FSH, **LH levels** typically rise during menopause due to reduced estrogen feedback, but an elevated LH is not part of the formal clinical diagnostic criteria for menopause.
- The **FSH-to-LH ratio** can also be used, but the 12-month amenorrhea rule remains the primary diagnostic standard.
*Pelvic ultrasound demonstrating decreased follicular activity*
- A pelvic ultrasound can show **decreased ovarian volume** and a lack of developing follicles, indicating reduced ovarian activity, but this is not a required diagnostic parameter for menopause.
- Clinical history and the 12-month cessation of menses are sufficient for diagnosis, making **imaging studies** generally unnecessary unless there are other concerns.
*Increased total cholesterol*
- While **elevated cholesterol levels** can be associated with menopause due to changes in estrogen, it is a metabolic consequence or associated risk factor, not a diagnostic criterion for menopause itself.
- Menopause can lead to an increased risk of **cardiovascular disease**, which includes dyslipidemia, but this is a secondary effect, not a primary diagnostic marker.
Bayesian approach to diagnosis US Medical PG Question 8: A 53-year-old man presents with a 2-year-history of dull, nonspecific flank pain that subsides with rest. His past medical history is significant for hypertension, hypercholesterolemia, and type 2 diabetes mellitus. He has no allergies and takes no medications. His father died of kidney disease at the age of 51, and his mother has been treated for ovarian cancer. On presentation, his blood pressure is 168/98 mm Hg, and his heart rate is 102/min. Abdominal examination is significant for palpable bilateral renal masses. His laboratory tests are significant for creatinine of 2.0 mg/dL and a BUN of 22 mg/dL. Which of the following tests is most recommended in this patient?
- A. Chest X-ray
- B. Genetic testing for polycystic kidney disease
- C. CT scan of abdomen and pelvis
- D. 24-hour urine protein collection
- E. Renal ultrasound (Correct Answer)
Bayesian approach to diagnosis Explanation: ***Renal ultrasound***
- This patient presents with classic features of **autosomal dominant polycystic kidney disease (ADPKD)**: bilateral palpable renal masses, hypertension, elevated creatinine, and a strong family history (father died of kidney disease at 51).
- **Renal ultrasound is the first-line imaging test** for diagnosing ADPKD due to its **non-invasiveness, no radiation exposure, high sensitivity for detecting cysts, and cost-effectiveness**.
- Ultrasound can establish the diagnosis using **established diagnostic criteria** (Pei-Ravine criteria based on age and number of cysts) and is recommended by **KDIGO guidelines** as the initial imaging modality.
- In this patient with clear clinical features and palpable masses, ultrasound will readily confirm the diagnosis by demonstrating multiple bilateral renal cysts.
*24-hour urine protein collection*
- This test quantifies **proteinuria** to assess for glomerular damage.
- While proteinuria can occur in ADPKD, it is not a diagnostic test and would not help identify or characterize the bilateral renal masses in this presentation.
*Chest X-ray*
- A chest X-ray evaluates the **lungs and heart**.
- It provides no diagnostic information regarding renal masses or kidney pathology and is not indicated in this case.
*Genetic testing for polycystic kidney disease*
- **Genetic testing** (for PKD1 or PKD2 mutations) can confirm ADPKD definitively and is useful for family counseling and cases with uncertain imaging findings.
- However, it is **not the first-line test** and is typically performed *after* imaging has established the diagnosis, or in specific situations (e.g., young patients, potential living kidney donors, atypical presentations).
- In this patient with clear clinical and anticipated imaging findings, genetic testing is unnecessary for initial diagnosis.
*CT scan of abdomen and pelvis*
- CT scan provides excellent anatomic detail and is useful in ADPKD for **evaluating complications** such as cyst hemorrhage, infection, suspected malignancy, or for **preoperative planning**.
- However, it is **not the first-line diagnostic test** due to higher cost, radiation exposure, and the fact that ultrasound is equally effective for initial diagnosis.
- CT would be reserved for situations where ultrasound is inconclusive or when complications are suspected.
Bayesian approach to diagnosis US Medical PG Question 9: A 50-year-old woman presents with severe abdominal pain. Past medical history is significant for a peptic ulcer. Physical examination is limited because the patient will not allow abdominal palpation due to the pain. The attending makes a presumptive diagnosis of peritonitis. Which of the following non-invasive maneuvers would be most helpful in confirming the diagnosis of peritonitis in this patient?
- A. Rectal examination shows guaiac positive stool
- B. Forced cough elicits abdominal pain (Correct Answer)
- C. Hyperactive bowel sounds are heard on auscultation
- D. Bowel sounds are absent on auscultation
- E. Pain is aroused with gentle intensity/pressure at the costovertebral angle
Bayesian approach to diagnosis Explanation: ***Forced cough elicits abdominal pain***
- A forced cough increases **intra-abdominal pressure**, which in turn stretches the inflamed peritoneum.
- Elicitation of pain with coughing is a highly sensitive and specific sign for **peritoneal irritation** and helps confirm the diagnosis of peritonitis.
*Rectal examination shows guaiac positive stool*
- **Guaiac positive stool** indicates the presence of blood in the stool, which is a sign of gastrointestinal bleeding.
- While a peptic ulcer can cause bleeding, this finding does not directly confirm **peritonitis** or peritoneal inflammation.
*Hyperactive bowel sounds are heard on auscultation*
- **Hyperactive bowel sounds** are often associated with conditions like gastroenteritis or partial bowel obstruction.
- In peritonitis, bowel sounds are typically diminished or absent due to **ileus**, not hyperactive.
*Bowel sounds are absent on auscultation*
- While **absent bowel sounds** can be a sign of peritonitis due to paralytic ileus, this finding is not as specific or immediately helpful as eliciting pain with coughing in confirming the primary diagnosis in a patient already presumed to have peritonitis.
- The absence of bowel sounds can also be seen in other conditions and may take longer to develop consistently.
*Pain is aroused with gentle intensity/pressure at the costovertebral angle*
- Pain at the **costovertebral angle (CVA)** typically indicates inflammation of the kidney or surrounding structures, such as in pyelonephritis.
- This finding is specific to **renal pathology** and not directly related to generalized peritonitis.
Bayesian approach to diagnosis US Medical PG Question 10: A 63-year-old retired teacher presents to his family physician for an annual visit. He has been healthy for most of his life and currently takes no medications, although he has had elevated blood pressure on several visits in the past few years but declined taking any medication. He has no complaints about his health and has been enjoying time with his grandchildren. He has been a smoker for 40 years–ranging from half to 1 pack a day, and he drinks 1 beer daily. On presentation, his blood pressure is 151/98 mm Hg in both arms, heart rate is 89/min, and respiratory rate is 14/min. Physical examination reveals a well-appearing man with no physical abnormalities. A urinalysis is performed and shows microscopic hematuria. Which of the following is the best next step for this patient?
- A. Perform intravenous pyelography
- B. Reassure the patient and recommend lifestyle modifications for his hypertension
- C. Perform a cystoscopy
- D. Perform a CT scan of the abdomen with contrast (Correct Answer)
- E. Repeat the urinalysis
Bayesian approach to diagnosis Explanation: ***Perform a CT scan of the abdomen with contrast***
- This patient has **asymptomatic microscopic hematuria** with significant risk factors: age >60 years and **40-year smoking history** (major risk factor for urothelial malignancy).
- According to **AUA guidelines**, patients with risk factors and microscopic hematuria require complete evaluation including **upper tract imaging (CT urography preferred) AND cystoscopy**.
- CT urography is the gold standard for detecting **renal cell carcinoma, urothelial carcinoma, and structural abnormalities** of the upper urinary tract.
- In high-risk patients like this, proceeding directly to imaging is appropriate rather than waiting to confirm persistent hematuria.
*Repeat the urinalysis*
- Repeating urinalysis is appropriate for patients with **transient causes** of hematuria (vigorous exercise, menstruation, urinary tract infection, recent instrumentation).
- This patient has **no evidence of transient causes** and has significant risk factors requiring immediate workup.
- Delaying evaluation in a high-risk patient could miss early malignancy when it is most treatable.
*Perform a cystoscopy*
- **Cystoscopy** is indeed part of the standard hematuria workup and evaluates the bladder and urethra for malignancy.
- However, it should be performed **in conjunction with upper tract imaging**, not alone, as it cannot visualize the kidneys or ureters.
- Both cystoscopy and imaging are needed for complete evaluation; imaging is typically done first to identify upper tract pathology.
*Reassure the patient and recommend lifestyle modifications for his hypertension*
- While addressing **hypertension** is important, ignoring microscopic hematuria in a high-risk patient is inappropriate.
- **Smoking and age** make this patient high-risk for genitourinary malignancies that require prompt investigation.
- This approach could result in dangerous delays in diagnosing serious conditions like **renal cell carcinoma or bladder cancer**.
*Perform intravenous pyelography*
- **Intravenous pyelography (IVP)** is an outdated imaging technique largely replaced by **CT urography**.
- CT urography provides superior sensitivity for detecting **renal masses and urothelial abnormalities** with better anatomical detail.
- IVP has lower diagnostic accuracy and higher radiation exposure compared to modern CT imaging.
More Bayesian approach to diagnosis US Medical PG questions available in the OnCourse app. Practice MCQs, flashcards, and get detailed explanations.
