Probability ranking in differentials US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Probability ranking in differentials. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Probability ranking in differentials US Medical PG Question 1: A group of gastroenterologists is concerned about low colonoscopy screening rates. They decide to implement a free patient navigation program to assist local residents and encourage them to obtain colonoscopies in accordance with U.S. Preventive Services Task Force (USPSTF) guidelines. Local residents were recruited at community centers. Participants attended monthly meetings with patient navigators and were regularly reminded that their adherence to screening guidelines was being evaluated. Colonoscopy screening rates were assessed via chart review, which showed that 90% of participants adhered to screening guidelines. Data collected via chart review for local residents recruited at community centers who did not participate in the free patient navigation system found that 34% of that population adhered to USPSTF guidelines. Which of the following has most likely contributed to the observed disparity in colonoscopy screening rates?
- A. Recall bias
- B. Confirmation bias
- C. Reporting bias
- D. Hawthorne effect (Correct Answer)
- E. Sampling bias
Probability ranking in differentials Explanation: ***Hawthorne effect***
- The **Hawthorne effect** is a type of reactivity in which individuals modify an aspect of their behavior in response to their awareness of being observed.
- In this study, participants were aware that their adherence to screening guidelines was being evaluated, likely leading to increased compliance simply due to this awareness rather than the efficacy of the patient navigation program alone.
*Recall bias*
- **Recall bias** occurs when participants disproportionately remember or inaccurately recall past events, often due to their current health status or beliefs.
- This bias is less likely here as colonoscopy screening rates were assessed via **chart review**, an objective measure, rather than participant self-report.
*Confirmation bias*
- **Confirmation bias** is the tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs or hypotheses.
- This bias typically affects the researchers or observers, not the participants' behavior in the observed manner, as the question focuses on the participants' increased screening rates.
*Reporting bias*
- **Reporting bias** refers to selective revealing or suppression of information during the reporting of research findings, and can occur when study participants selectively report symptoms or behaviors.
- While participants might selectively report, the data here was gathered through **chart review**, which is a more objective measure of actual behavior, making reporting bias less likely to explain the disparity in screening rates.
*Sampling bias*
- **Sampling bias** occurs when a sample is not representative of the population from which it is drawn, leading to skewed results.
- While there might be some sampling bias in who chose to participate in the free program, the observed disparity is specifically about behavior change in those *being observed*, pointing more strongly to the Hawthorne effect.
Probability ranking in differentials US Medical PG Question 2: A 17-year-old man is brought by his mother to his pediatrician in order to complete medical clearance forms prior to attending college. During the visit, his mother asks about what health risks he should be aware of in college. Specifically, she recently saw on the news that some college students were killed by a fatal car crash. She therefore asks about causes of death in this population. Which of the following is true about the causes of death in college age individuals?
- A. More of them die from homicide than suicide
- B. More of them die from suicide than injuries
- C. More of them die from cancer than suicide
- D. More of them die from homicide than injuries
- E. More of them die from homicide than cancer (Correct Answer)
Probability ranking in differentials Explanation: ***More of them die from homicide than cancer***
- While relatively rare, **homicide rates** for college-aged individuals (18-24 years) are generally higher than their rates of death due to **cancer**.
- **Cancer** is a leading cause of death in older populations but is much less common in young adults.
*More of them die from homicide than suicide*
- **Suicide** is a significantly more common cause of death than homicide among college-aged individuals.
- Data consistently shows that **suicide** ranks as one of the top causes of death in this demographic, often second only to unintentional injuries.
*More of them die from suicide than injuries*
- **Unintentional injuries** (including motor vehicle accidents, accidental poisoning, and falls) are the leading cause of death in the 18-24 age group.
- **Suicide** is typically the second leading cause, meaning more individuals die from injuries than from suicide.
*More of them die from cancer than suicide*
- As mentioned, **suicide** is a much more prevalent cause of death in young adults than cancer.
- **Cancer deaths** are relatively uncommon in this age group compared to other causes like injuries and suicide.
*More of them die from homicide than injuries*
- **Unintentional injuries** are the leading cause of death among college-aged individuals.
- **Homicide rates** are considerably lower than injury rates in this population.
Probability ranking in differentials US Medical PG Question 3: 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)
Probability ranking in differentials 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.
Probability ranking in differentials 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)
Probability ranking in differentials 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**.
Probability ranking in differentials US Medical PG Question 5: A 57-year-old man presents to the emergency department for weight loss and abdominal pain. The patient states that he has felt steadily more fatigued over the past month and has lost 22 pounds without effort. Today, he fainted prompting his presentation. The patient has no significant past medical history. He does have a 33 pack-year smoking history and drinks 4 to 5 alcoholic drinks per day. His temperature is 99.5°F (37.5°C), blood pressure is 100/58 mmHg, pulse is 100/min, respirations are 17/min, and oxygen saturation is 98% on room air. On physical exam, you see a patient who is very thin and appears to be pale. Stool fecal occult blood testing is positive. A CT scan of the abdomen is performed demonstrating a mass in the colon with multiple metastatic lesions scattered throughout the abdomen. The patient is informed of his diagnosis of metastatic colon cancer. When the patient conveys the information to his family he focuses his efforts on discussing the current literature in the field and the novel therapies that have been invented. He demonstrates his likely mortality outcome which he calculated using the results of a large multi-center study. Which of the following is this patient most likely demonstrating?
- A. Intellectualization (Correct Answer)
- B. Dissociation
- C. Rationalization
- D. Optimism
- E. Pessimism
Probability ranking in differentials Explanation: ***Intellectualization***
- This defense mechanism involves **focusing on the intellectual aspects** of a stressful situation, using logical reasoning and factual analysis to avoid experiencing distressing emotions.
- The patient demonstrates this by discussing **literature, novel therapies, and mortality statistics** regarding his metastatic colon cancer.
*Dissociation*
- **Dissociation** involves a mental process that causes a lack of connection in a person's thoughts, memory, and sense of identity.
- This patient is actively engaging with the information, not disconnecting from it.
*Rationalization*
- **Rationalization** is creating logical but false explanations for unacceptable thoughts, feelings, or behaviors to justify them.
- The patient is not trying to justify his actions or feelings, but rather to understand his disease intellectually.
*Optimism*
- **Optimism** is a disposition to look on the favorable side of events or conditions and to expect the most favorable outcome.
- While hope for novel therapies could be seen as optimistic, his detailed calculation of mortality outcomes is a realistic, rather than purely optimistic, approach.
*Pessimism*
- **Pessimism** is a tendency to see the worst aspect of things or believe that the worst will happen.
- The patient is engaging with the facts of his diagnosis, even calculating his mortality outcome, which is not necessarily a pessimistic but rather a realistic and intellectual approach.
Probability ranking in differentials US Medical PG Question 6: A female presents with a 1 × 1 cm thyroid swelling. What is the next best step in management?
- A. I-131
- B. TSH (Correct Answer)
- C. TSH & T4
- D. T3 & T4
- E. FNAC
Probability ranking in differentials Explanation: ***Correct Option: TSH***
- **Thyroid-stimulating hormone (TSH)** is the most sensitive initial test to assess thyroid function when a thyroid nodule is discovered.
- An abnormal TSH level (either high or low) can guide further investigation into whether the nodule is associated with a functional thyroid disorder.
- **TSH should be the first test** according to American Thyroid Association guidelines for thyroid nodule evaluation.
*Incorrect Option: I-131*
- **I-131 (radioactive iodine therapy)** is a treatment modality for hyperthyroidism or thyroid cancer, not a diagnostic step for initial thyroid swelling evaluation.
- Administering I-131 before assessing thyroid function would be inappropriate and could lead to unnecessary or harmful intervention.
*Incorrect Option: TSH & T4*
- While TSH is crucial, adding **T4 (thyroxine)** as an initial step is often not necessary if TSH is normal, as TSH alone effectively screens for primary thyroid dysfunction.
- Measuring both TSH and T4 is typically reserved for situations where TSH is abnormal or when central hypothyroidism is suspected.
*Incorrect Option: T3 & T4*
- Measuring **T3 (triiodothyronine)** along with T4 as an initial screening for a thyroid nodule is generally not recommended.
- T3 levels are primarily used to diagnose **hyperthyroidism** or to evaluate the severity of thyrotoxicosis after an abnormal TSH and T4 have been identified.
*Incorrect Option: FNAC*
- While **Fine Needle Aspiration Cytology (FNAC)** is an essential diagnostic tool for thyroid nodules, it is typically performed after TSH assessment.
- FNAC is indicated for nodules >1 cm with suspicious ultrasound features, but **functional assessment with TSH comes first** to rule out hyperfunctioning nodules.
Probability ranking in differentials US Medical PG Question 7: What is the most appropriate initial investigation for a solitary thyroid nodule (STN)?
- A. I-123 scan
- B. Ultrasound (Correct Answer)
- C. Fine-needle aspiration (FNA) biopsy
- D. Thyroid function tests (TFTs)
- E. CT scan of the neck
Probability ranking in differentials Explanation: ***Ultrasound***
- **Ultrasound** is the initial investigation of choice for a solitary thyroid nodule (STN) because it can differentiate between **solid, cystic, or mixed lesions**, assess nodule size, and identify suspicious features (e.g., microcalcifications, irregular margins, internal vascularity).
- It also helps to determine if there are other nodules not palpable on physical examination, allowing for a more complete assessment of the **thyroid gland**.
*Fine-needle aspiration (FNA) biopsy*
- **FNA biopsy** is the most accurate diagnostic tool for evaluating the malignant potential of a thyroid nodule, but it is typically performed *after* an initial ultrasound has characterized the nodule.
- It requires guidance (often by ultrasound) to obtain an adequate sample for cytological analysis, making ultrasound a prerequisite for optimal FNA performance.
*Thyroid function tests (TFTs)*
- **TFTs (TSH, T3, T4)** are important for assessing the functional status of the thyroid gland (e.g., hyperthyroidism or hypothyroidism) and can provide context for the nodule.
- However, TFTs do not directly evaluate the **morphology or malignant potential** of the nodule itself, making them less appropriate as an initial, stand-alone investigation for an STN.
*I-123 scan*
- An **I-123 scan** (radioactive iodine uptake and scan) is used to determine if a nodule is "hot" (hyperfunctioning/benign) or "cold" (non-functioning/potentially malignant).
- It is typically reserved for cases where **TSH levels are suppressed**, suggesting a hyperfunctioning nodule, and is not the first-line imaging modality for initial characterization of all STNs.
*CT scan of the neck*
- **CT scan** can visualize thyroid nodules and assess for extrathyroidal extension or lymphadenopathy, but it is **not recommended as an initial investigation** for STN.
- It involves **radiation exposure**, is more expensive than ultrasound, and provides **less detailed characterization** of nodule morphology compared to ultrasound, making it a less appropriate first-line modality.
Probability ranking in differentials US Medical PG Question 8: An 11-year-old boy is brought to the emergency department 30 minutes after he was found screaming and clutching his head. He has had nausea and occasional episodes of vomiting for 1 week, fever and left-sided headaches for 2 weeks, and increasing tooth pain over the past 3 weeks. He has no history of ear or sinus infections. He is in moderate distress. His temperature is 38.7°C (101.7°F), pulse is 170/min, respirations are 19/min, and blood pressure is 122/85 mmHg. He is confused and only oriented to person. The pupils react sluggishly to light. Fundoscopic examination shows papilledema bilaterally. Extraocular movements are normal. Flexion of the neck causes hip flexion. Which of the following is the most likely diagnosis?
- A. Subarachnoid hemorrhage
- B. Cavernous sinus thrombosis
- C. Pyogenic brain abscess (Correct Answer)
- D. HSV encephalitis
- E. Medulloblastoma
Probability ranking in differentials Explanation: ***Pyogenic brain abscess***
- The patient's history of **increasing tooth pain** over three weeks, followed by fever, headache, nausea, vomiting, confusion, and **papilledema**, strongly suggests a pyogenic brain abscess originating from a dental infection.
- **Brudzinski's sign** (nuchal rigidity with **hip flexion upon neck flexion**) indicates meningeal irritation, and confusion with sluggish pupillary reaction are signs of increased intracranial pressure, consistent with an expanding mass lesion and inflammation.
*Subarachnoid hemorrhage*
- While it can cause sudden severe headache ("thunderclap"), nausea, vomiting, and meningeal signs, the **subacute onset** of symptoms (weeks) and the presence of prior dental pain make this diagnosis less likely.
- **Fever** with prolonged, progressive symptoms and signs of focal neurological deficits (which can be subtle like confusion and sluggish pupils) are not typical for SAH.
*Cavernous sinus thrombosis*
- This condition is typically associated with infections in the facial region or sinuses, leading to **ophthalmoplegia**, **proptosis**, and chemosis due to involvement of cranial nerves III, IV, VI, and the ophthalmic/maxillary branches of V.
- Although the patient has headache and fever, the absence of specific ocular signs like paralysis of extraocular muscles or proptosis makes this less probable.
*HSV encephalitis*
- While HSV encephalitis can cause fever, headache, altered mental status, and seizures, its onset is typically **acute to subacute** (days), and it often presents with **focal neurological deficits** or personality changes, not typically originating from dental pain.
- The preceding tooth pain and the relatively prolonged symptom timeline (weeks) are less characteristic of primary HSV encephalitis.
*Medulloblastoma*
- This is a common posterior fossa tumor in children, which can cause symptoms of increased ICP like headache, nausea, vomiting, and papilledema due to hydrocephalus.
- However, the presence of **fever** and a clear preceding **infectious source** (dental pain) points away from a primary tumor and more towards an infectious process like an abscess.
Probability ranking in differentials US Medical PG Question 9: A 70-year-old man is brought to the emergency department for the evaluation of worsening upper abdominal pain that he first noticed this morning after waking up. The pain is of tearing and burning quality and radiates to his back. Yesterday, he underwent an upper endoscopy and was diagnosed with gastritis and a large hiatal hernia. He has hypertension, hypercholesteremia, and a left bundle branch block that was diagnosed 5 years ago. The patient's mother died of myocardial infarction at the age of 70 years, and his father died of aortic dissection at the age of 65 years. The patient smoked one pack of cigarettes daily for the past 40 years, but quit 10 years ago. He drinks three beers daily. Current medications include hydrochlorothiazide, amlodipine, atorvastatin, and pantoprazole. The patient appears to be in mild distress. His temperature is 37.8°C (100.4°F), pulse is 103/min, and blood pressure is 135/89 mm Hg in the left arm and 132/90 mm Hg in the right arm. Cardiopulmonary examination shows crackling with every heartbeat. Abdominal examination shows tenderness to palpation in the epigastric region; bowel sounds are normal. Laboratory studies show:
Hemoglobin 16.0 g/dL
Leukocyte count 11,000/mm3
Na+ 140 mEq/L
K+ 4.2 mEq/L
Cl- 101 mEq/L
HCO3- 25 mEq/L
Creatinine 1.3 mg/dL
Alanine aminotransferase 21 U/L
Aspartate aminotransferase 43 U/L
Lipase 40 U/L (N = 14–280)
Troponin I 0.025 ng/mL (N < 0.1)
Chest x-ray shows a large hiatal hernia and mediastinal lucency. A 12-lead EKG shows sinus tachycardia and a left bundle branch block. Which of the following is the most appropriate next step in diagnosis?
- A. Contrast esophagography with gastrografin (Correct Answer)
- B. Contrast-enhanced CT of the aorta
- C. Esophagogastroduodenoscopy
- D. Abdominal ultrasound
- E. Coronary angiography
Probability ranking in differentials Explanation: ***Contrast esophagography with gastrografin***
- The patient's symptoms (worsening abdominal pain after endoscopy, tearing/burning quality radiating to the back) and imaging findings (mediastinal lucency on chest x-ray, crackling with every heartbeat suggestive of **Hamman's sign**) are highly indicative of **esophageal perforation**.
- **Gastrografin** (water-soluble contrast) is the preferred initial study for suspected esophageal perforation because it is less irritating to the mediastinal tissues and can be reabsorbed if extravasated, unlike barium.
*Contrast-enhanced CT of the aorta*
- Although the patient has risk factors for **aortic dissection** (hypertension, family history), his symptoms are more classic for esophageal perforation, particularly after recent endoscopy.
- The **mediastinal lucency** on chest x-ray strongly suggests esophageal perforation rather than aortic dissection.
*Esophagogastroduodenoscopy*
- Repeating an **EGD** would be contraindicated and dangerous in a patient with suspected esophageal perforation as it could worsen the tear and lead to further mediastinal contamination.
- The initial EGD likely contributed to the current presumed perforation.
*Abdominal ultrasound*
- An abdominal ultrasound is useful for evaluating conditions like **cholecystitis**, **pancreatitis**, or abdominal aortic aneurysm, but it would not reliably detect an esophageal perforation.
- The patient's pain radiation and chest x-ray findings point away from these diagnoses.
*Coronary angiography*
- While the patient has cardiac risk factors and a family history of **myocardial infarction**, his **troponin I** is normal, and his pain description of tearing/burning quality radiating to the back is less typical for cardiac ischemia.
- The combined clinical picture and mediastinal lucency point strongly towards esophageal pathology.
Probability ranking in differentials US Medical PG Question 10: A 43-year-old woman was admitted to the hospital after a fall. When the emergency services arrived, she was unresponsive, did not open her eyes, but responded to painful stimuli. The witnesses say that she had convulsions lasting about 30 seconds when she lost consciousness after a traumatic event. On her way to the hospital, she regained consciousness. On admission, she complained of intense headaches and nausea. She opened her eyes spontaneously, was responsive but confused, and was able to follow motor commands. Her vital signs are as follows: blood pressure, 150/90 mm Hg; heart rate, 62/min; respiratory rate, 13/min; and temperature, 37.3℃ (99.1℉). There are no signs of a skull fracture. The pupils are round, equal, and poorly reactive to light. She is unable to fully abduct both eyes. Ophthalmoscopy does not show papillary edema or retinal hemorrhages. She has nuchal rigidity and a positive Kernig sign. An urgent head CT does not show any abnormalities. Which of the following is a proper investigation to perform in this patient?
- A. Lumbar puncture (Correct Answer)
- B. Angiography
- C. Sonography
- D. Brain MRI
- E. EEG
Probability ranking in differentials Explanation: ***Lumbar puncture***
- The patient's symptoms, including **severe headache**, **nuchal rigidity**, **positive Kernig sign**, and **abnormal oculomotor findings** (poorly reactive pupils, inability to abduct eyes), despite a normal head CT, are highly suggestive of **subarachnoid hemorrhage**. A lumbar puncture is essential to look for **xanthochromia** (yellowish discoloration of CSF due to bilirubin from lysed red blood cells), which confirms the diagnosis, especially when CT is negative in the first 6-12 hours.
- The history of a "traumatic event" followed by convulsions and transient loss of consciousness, along with a "lucid interval" then renewed symptoms (headache, confusion), raises suspicion for head injury leading to hemorrhagic event. The **elevated BP and bradycardia** (Cushing's reflex components), though not fully developed, also suggest increased intracranial pressure, further warranting investigation for hemorrhage.
*Angiography*
- While angiography (CTA or conventional) is performed **after a subarachnoid hemorrhage is confirmed** to identify the source of bleeding (e.g., aneurysm), it is not the *initial* diagnostic test to *confirm* the hemorrhage itself when CT is negative.
- Doing an angiography before ruling out significant hemorrhage via LP (when CT is negative) is premature and could expose the patient to unnecessary risks without a confirmed diagnosis.
*Sonography*
- **Sonography** (ultrasound) has **no role** in the acute diagnosis of subarachnoid hemorrhage or other intracranial pathology in adults.
- It is used for imaging the brain in neonates through the open fontanelles but is ineffective through the adult skull.
*Brain MRI*
- While an **MRI is more sensitive than CT for detecting subarachnoid hemorrhage** (especially subacute hemorrhage or small bleeds missed by CT), it is generally **less readily available** in an emergency setting than CT and LP.
- In cases where CT is negative but clinical suspicion for SAH is high, **lumbar puncture is typically the next step** as it can detect early SAH via xanthochromia, which might not be immediately visible on MRI. MRI may be used later to identify causes or small bleeds not picked up by CT.
*EEG*
- An **EEG (electroencephalogram)** is used to **evaluate seizure activity** and other types of brain dysfunction related to electrical activity.
- While the patient had convulsions, the primary concern given her overall presentation (severe headache, meningeal signs, altered mental status, and cranial nerve palsies) is **subarachnoid hemorrhage**, not solely seizure. An EEG would not help diagnose the underlying cause of her acute neurological deterioration.
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