Differential diagnosis

Differential 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)

Differential 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.

Differential diagnosis US Medical PG Question 2: A group of neurologists develop a new blood test for Alzheimer's. They are optimistic about the test, as they have found that for any given patient, the test repeatedly produces very similar results. However, they find that the new test results are not necessarily consistent with the gold standard of diagnosis. How would this new test most accurately be described?

• A. Valid and reliable
• B. Reliable (Correct Answer)
• C. Valid
• D. Biased
• E. Neither valid nor reliable

Differential diagnosis Explanation: ***Reliable*** - The test produces **similar results repeatedly** upon repeated measures, indicating high **reliability** or **precision**. - Reliability refers to the **consistency** of a measure, even if it is not accurate. *Valid and reliable* - While the test is **reliable**, it is explicitly stated that the results are **not consistent with the gold standard**, meaning it lacks **validity**. - A test must be both **consistent** (reliable) and **accurate** (valid) to be described as valid and reliable. *Valid* - **Validity** refers to the **accuracy** of a test, or how well it measures what it is supposed to measure. - The test is explicitly stated to **not be consistent with the gold standard**, indicating a lack of agreement with the true measure of Alzheimer's. *Biased* - **Bias** refers to a **systematic error** in measurement that can lead to consistently high or low results compared to the true value. - While the test might be biased due to its lack of consistency with the gold standard, "biased" is not the most accurate single descriptor of its measurement properties given the information provided. *Neither valid nor reliable* - The test is described as producing **very similar results repeatedly**, which directly indicates it has **high reliability**. - Therefore, stating it is neither valid nor reliable is incorrect, as it possesses reliability.

Differential diagnosis US Medical PG Question 3: A research team develops a new monoclonal antibody checkpoint inhibitor for advanced melanoma that has shown promise in animal studies as well as high efficacy and low toxicity in early phase human clinical trials. The research team would now like to compare this drug to existing standard of care immunotherapy for advanced melanoma. The research team decides to conduct a non-randomized study where the novel drug will be offered to patients who are deemed to be at risk for toxicity with the current standard of care immunotherapy, while patients without such risk factors will receive the standard treatment. Which of the following best describes the level of evidence that this study can offer?

• A. Level 1
• B. Level 3 (Correct Answer)
• C. Level 5
• D. Level 4
• E. Level 2

Differential diagnosis Explanation: ***Level 3*** - A **non-randomized controlled trial** like the one described, where patient assignment to treatment groups is based on specific characteristics (risk of toxicity), falls into Level 3 evidence. - This level typically includes **non-randomized controlled trials** and **well-designed cohort studies** with comparison groups, which are prone to selection bias and confounding. - The study compares two treatments but lacks randomization, making it Level 3 evidence. *Level 1* - Level 1 evidence is the **highest level of evidence**, derived from **systematic reviews and meta-analyses** of multiple well-designed randomized controlled trials or large, high-quality randomized controlled trials. - The described study is explicitly stated as non-randomized, ruling out Level 1. *Level 2* - Level 2 evidence involves at least one **well-designed randomized controlled trial** (RCT) or **systematic reviews** of randomized trials. - The current study is *non-randomized*, which means it cannot be classified as Level 2 evidence, as randomization is a key criterion for this level. *Level 4* - Level 4 evidence includes **case series**, **case-control studies**, and **poorly designed cohort or case-control studies**. - While the study is non-randomized, it is a controlled comparative trial rather than a case series or retrospective case-control study, placing it at Level 3. *Level 5* - Level 5 evidence is the **lowest level of evidence**, typically consisting of **expert opinion** without explicit critical appraisal, or based on physiology, bench research, or animal studies. - While the drug was initially tested in animal studies, the current human comparative study offers a higher level of evidence than expert opinion or preclinical data.

Differential diagnosis US Medical PG Question 4: 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

Differential diagnosis 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.

Differential diagnosis US Medical PG Question 5: A 52-year-old man comes to the physician because of a 3-week history of a cough and hoarseness. He reports that the cough is worse when he lies down after lunch. His temperature is 37.5°C (99.5°F); the remainder of his vital signs are within normal limits. Because the physician has recently been seeing several patients with the common cold, the diagnosis of a viral upper respiratory tract infection readily comes to mind. The physician fails to consider the diagnosis of gastroesophageal reflux disease, which the patient is later found to have. Which of the following most accurately describes the cognitive bias that the physician had?

• A. Framing
• B. Anchoring
• C. Visceral
• D. Confirmation
• E. Availability (Correct Answer)

Differential diagnosis Explanation: ***Availability*** - The physician recently seeing several patients with the common cold led to this diagnosis readily coming to mind, demonstrating how easily recalled examples can disproportionately influence diagnosis. - This bias occurs when easily recalled instances or information (like recent cases of common cold) are used to estimate the likelihood or frequency of an event, even if other more relevant data exist. *Framing* - This bias occurs when the way information is presented (e.g., as a gain or a loss) influences a decision, rather than the intrinsic characteristics of the options themselves. - The scenario does not involve the presentation of information in different ways to sway the physician's judgment. *Anchoring* - This bias involves relying too heavily on an initial piece of information (the "anchor") when making subsequent judgments, often leading to insufficient adjustment away from that anchor. - While the physician initially considered a viral URI, the setup is more about the ease of recall influencing the decision rather than being stuck on an initial data point. *Visceral* - This is not a commonly recognized cognitive bias in the context of medical decision-making; "visceral" largely refers to emotional or intuitive feelings rather than a structured cognitive bias. - Cognitive biases describe systematic patterns of deviation from norm or rationality in judgment, not merely emotional responses. *Confirmation* - This bias involves seeking, interpreting, favoring, and recalling information in a way that confirms one's pre-existing beliefs or hypotheses. - The physician did not actively seek information to confirm the common cold diagnosis; rather, the diagnosis came to mind due to recent encounters, which aligns with availability bias.

Differential diagnosis Flashcard 1 of 5

Question: A positive _____ sign is when minor pressure induces skin separation

Answer: Nikolsky

Differential diagnosis Flashcard 2 of 5

Question: What is the differential diagnosis (2) in a Cushing syndrome patient with elevated ACTH?_____

Answer: 1. Cushing disease (ACTH-secreting pituitary adenoma) 2. Ectopic ACTH secretion

Differential diagnosis Flashcard 3 of 5

Question: What pathologies (2) are characterized by hematuria with no casts? _____

Answer: Bladder cancer, Kidney stones

Differential diagnosis Flashcard 4 of 5

Question: The following signs and symptoms are characteristic of what diesase: broad-based ataxia, positive Romberg, Argyll-Robertson pupil, Charcot joint, stroke

Answer: tertiary syphilis

Differential diagnosis Flashcard 5 of 5

Question: The following signs and symptoms are characteristic of what diesase: constitutional symptoms, maculopapuar rash (especially on palms and soles)

Answer: secondary syphilis