WBR1015

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Serge Korjian)]]
Exam Type	USMLE Step 1
Main Category	Physiology
Sub Category	Cardiology
Prompt	[[Prompt::A pharmaceutical company is developing a new anti-arrhythmic drug that targets the slow voltage gated potassium channels. During which of the following phases of the action potential (depicted below) is this drug most active? ]]
Answer A	A
Answer A Explanation	A corresponds to phase 0 of the ventricular action potential. Phase 0 is characterized by a depolarization caused by the abrupt opening of the voltage gated sodium channel.
Answer B	B
Answer B Explanation	B corresponds to phase 1 of the ventricular action potential. Phase 1 is characterized by an early repolarization caused by the closure of the sodium channels and the opening of voltage gated potassium channels.
Answer C	C
Answer C Explanation	C corresponds to phase 2 of the ventricular action potential. Phase 2 is characterized by a plateau. The plateau results from the opening of the voltage gated calcium channels leading to an influx of calcium that balances the potassium efflux.
Answer D	D
Answer D Explanation	D corresponds to phase 3 of the ventricular action potential. Phase 3 is characterized by a rapid repolarization caused by the closure of the calcium channels and opening of the slow voltage gated potassium channel.
Answer E	E
Answer E Explanation	E corresponds to phase 4 of the ventricular action potential. Phase 4 is characterized by a resting potential which is caused by the high potassium permeability through the potassium channels.
Right Answer	D
Explanation	[[Explanation::The ventricular action potential is composed of four phases: Phase 0 (depolarization): abrupt opening of the voltage gated sodium channel Phase 1 (early repolarization): closure of the sodium channels and opening of voltage gated potassium channels Phase 2 (plateau): opening of the voltage gated calcium channels leading to an influx of calcium that balances the potassium efflux Phase 3 (rapid repolarization): closure of the calcium channels and opening of the slow voltage gated potassium channel Phase 4 (resting potential): high potassium permeability through the potassium channels The image below details the different phases of the ventricular action potential. Educational Objective: The phase 3 of the ventricular action potential is characterized by a rapid repolarization caused by the closure of the calcium channels and opening of the slow voltage gated potassium channel. References: First Aid for USMLE Step 1, 2014 page 274]]
Approved	Yes
Keyword	Action potential, Antiarrhythmics, Phase 3, Calcium channels, Potassium chanels, Mechanism of action
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