**Question:** Which of the following is not true of "a" wave of venous pulsations in neck?

A. The "a" wave represents the systolic phase of the cardiac cycle
B. The "a" wave is caused by blood being ejected from the heart
C. The "a" wave is not present in all veins of the neck
D. The "a" wave is a result of the cardiac contractility

**Core Concept:** The "a" wave, also known as the systolic phase or positive wave, is a component of venous pulsations in the neck and represents the systolic phase of the cardiac cycle. During this phase, blood is ejected from the heart into the circulatory system, causing an increase in pressure in the venous system, which results in the "a" wave.

**Why the Correct Answer is Right:** A correct understanding of the cardiac cycle is crucial to understanding the "a" wave. The "a" wave occurs because the heart contracts (systole) and ejects blood, increasing pressure in the venous system. This pressure increase results in a positive wave, which is also known as the systolic phase.

**Why Each Wrong Option is Incorrect:**

A. The systolic phase (or "a" wave) is indeed caused by the contraction of the heart (systole), but the statement "The 'a' wave represents the systolic phase of the cardiac cycle" is correct, making option A incorrect.

B. Although blood ejection from the heart contributes to the "a" wave, the entire statement "The 'a' wave is caused by blood being ejected from the heart" is not entirely accurate. The "a" wave is primarily caused by the systolic phase (cardiac contraction) and the increased pressure in the venous system.

C. The statement "The 'a' wave is present in all veins of the neck" is incorrect, as the "a" wave can be absent in some veins due to factors such as venous valves or variations in anatomy. Therefore, option C is incorrect.

D. The "a" wave is caused by the systolic phase (cardiac contraction) and increased pressure in the venous system, not just heart contractility. So, option D is incorrect.

**Clinical Pearl:**

The "a" wave in venous pulsations reflects the interplay between cardiac contraction (systole) and the resulting pressure increase in the venous system. It is essential to differentiate between the heart's systolic phase, venous pressure increase, and the specific venous system. Understanding venous pulsations and their relationship to cardiac contraction is essential for clinical reasoning and diagnosis.