Which of the following is a phase of the cardiac cycle?

• A. Atrial systole
• B. Ventricular diastole
• C. Atrioventricular conduction
• D. Cardiac muscle refractory state

Answer: (A)

The main idea is recognizing the distinct mechanical events that make up the heart’s cycle. Atrial systole is a specific phase where the atria contract to push blood into the ventricles, finishing ventricular filling just before the ventricles themselves contract. This is a defined, time-bound part of the cycle you can identify on the heartbeat sequence and physiology.

Why this is the best choice: it directly represents a concrete phase of the cardiac cycle—the atria actively contract as one coordinated stage, contributing to the sequence of events that drive circulation.

The other options point to non-kinematic aspects of heart function. Atrioventricular conduction is the electrical pathway that coordinates timing, not a mechanical phase of contraction or relaxation. The cardiac muscle refractory state describes a property during the action potential, not a phase of the cycle. Ventricular diastole is indeed a phase, but the example here clearly illustrating a defined cycle phase is atrial systole.