WBR0038
| Author | PageAuthor::Gonzalo Romero |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Behavioral Science/Psychiatry |
| Sub Category | SubCategory::Infectious Disease |
| Prompt | [[Prompt::A study is trying to prove different parameters of a new test used in diagnosing an infectious endemic disease in developing countries. Underserved areas’ tribes have have called the disease “Ztika”. For years physicians have struggled to diagnose it early enough to treat it soon so the infectious organism is controlled with antimicrobial therapy with the final goal of decreasing mortality. The disease is asymptomatic until severe symptoms develop and is exactly when the immune response is triggered making the final diagnosis possible through IgM specific serologic testing. A group of investigators had the initiative to develop a new test X-22231 which screens for the serum marker Z-AWS (a component of the infectious organism), present in early phases of this infectious disease called “Ztika” and decide to run a Phase I trial which proved to have promising results. The researchers move onto a Phase II trial recruiting more patients. The Institutional Review Board meets with the Sponsors and Data Monitoring Safety Committee in order to establish that a proper study design is being created and conducted. The study starts having an initial sample which involves 500 subjects and followed overtime which helped gathering data. The pre-test probabilities were determined; Sensitivity 90% and Specificity 80%. It was determined that 100 patients did have the disease “Ztika”. What is the probability that a patient with a positive result from the X-22231 test really had the disease?]] |
| Answer A | AnswerA::90/100 |
| Answer A Explanation | [[AnswerAExp::Sensitivity TP/ (TP + FN)]] |
| Answer B | AnswerB::320/ 400 |
| Answer B Explanation | [[AnswerBExp::Specificity or TN / (TN + FP)]] |
| Answer C | AnswerC::90/170 |
| Answer C Explanation | AnswerCExp::PPV. Correct answer. See the explanation above. |
| Answer D | AnswerD::320 / 330 |
| Answer D Explanation | [[AnswerDExp::Negative Predictive Value (NPV); or the probability that a negative test result is truly negative. TN / (TN + FN)]] |
| Answer E | AnswerE::90 + 320 / 500 |
| Answer E Explanation | [[AnswerEExp::Accuracy “Trues divided by the sample size” or (TN + TP) / (TP + TN + FP + FN)]] |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::Test taking strategy tip: It is important to read throughly this question scanning for the data that is vital which will help in answering the question correctly. USMLE Step 1 more often is including long stems, so being able to read quickly, yet carefully will enable the examinee to screen for the information necessary to aswer correctly. This question tests for basic statistical knowledge which is commony tested on STEP 1 and other USMLE examinations. A great tip is to read at the final lines to find what the question is really looking for.
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| Approved | Approved::Yes |
| Keyword | |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |