Arrhythmias USMLE Step 1 Practice Question
A 22-year-old medical student presents with recurrent episodes of sinus tachycardia (HR 115 bpm) triggered by stress. Vital signs show BP 128/76, RR 16, temp 37°C, SpO2 98%. ECG demonstrates normal P-wave morphology with shortened PR intervals. Troponin is normal. He denies palpitations at rest. After starting metoprolol, tachycardic episodes resolve. Which phase of the sinoatrial nodal action potential is most directly affected by decreased sympathetic stimulation?
Answer choices
- APhase 1 early repolarization in atrial cells
- BAbsolute refractory period of skeletal muscle
- CPhase 0 depolarization in ventricular myocytes
- DSlope of phase 4 spontaneous depolarization in pacemaker cellsCorrect answer
- EPlateau phase of Purkinje fibers
- FMaximum diastolic potential of sinoatrial nodal pacemaker cells
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