I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
Q- A 65-year-old woman is brought to the emergency department because of a 10-minute history of chest tightness and severe pain of her left arm. Physical examination shows jugular venous distention. Crackles are heard over the lung fields. An ECG shows ST-segment elevation greater than 1 mm in leads V4 through V6 and new Q waves. Serum studies show an increased troponin I concentration. Which of the following labeled points in the graph best represents the changes in cardiac function that occurred during the first 10 seconds after the onset of pain in this patient?
(A) W → X
(B) W → Y
(C) W → Z
(D) X → W
(E) X → Y
(F) X → Z
(G) Z → W
(H) Z → X
(I) Z → Y
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Answer
In the case of an acute myocardial infarction (MI), especially within the first few seconds to minutes, Cardiac output decreases due to reduced myocardial contractility. End-diastolic volume increases because blood backs up as the heart struggles to pump effectively.
Interpreting W and Z on the Graph
- Point W represents normal function before the onset of MI.
- Point Z shows a state with lower cardiac output and higher end-diastolic volume compared to W, which aligns with the immediate effects of an MI.
So, moving from W to Z effectively represents a decrease in cardiac output and an increase in end-diastolic volume.
The correct answer is C