USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

Which conditions can cause spontaneous depolarization leading to tachyarrhythmias by causing spontaneous depolarization of ventricles in phase 3 or 4?

1. Hypercalcemia

2. Hypermagnesemia

3. Hypokalemia

4. Prolonged PR interval

The correct answer is: Hypokalemia

Spontaneous depolarization leading to tachyarrhythmias can occur due to changes in electrolytes that affect cardiac action potentials. Hypokalemia, or low potassium levels, is particularly significant because it can lead to an increase in the resting membrane potential, making the cells more depolarized. This change can result in early afterdepolarizations, especially during the repolarization phases (phase 3) of the cardiac action potential. These afterdepolarizations can trigger ectopic beats or disrupt the normal rhythm, ultimately leading to tachyarrhythmias. Potassium plays a critical role in maintaining the stability of the resting membrane potential and ensuring proper repolarization of cardiac myocytes. Other options, such as hypercalcemia, can also influence cardiac rhythms and may cause increased contractility or other types of rhythm disturbances, but they do so via different mechanisms and are not directly associated with spontaneous depolarization in phase 3 or 4. Hypermagnesemia typically has a stabilizing effect on cardiac membrane potentials, and a prolonged PR interval is indicative of a delay in conduction, not spontaneous depolarization in the ventricles. Thus, hypokalemia is the condition most associated with spontaneous depolarization leading to