USMLE-Rx Step 1 Qmax Challenge #3990

Check out today’s Step 1 Qmax Question Challenge.

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A patient with atrial fibrillation is started on a drug after failing first-line therapies. His physician is careful to monitor for adverse effects such as pulmonary fibrosis and thyroid abnormalities. The diagram illustrates the cardiac action potential.

During which phase does this drug have its most significant clinical effect?

A. Phase 0
B. Phase 1
C. Phase 2
D. Phase 3
E. Phase 4

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