(D) contraction of left ventricle
When the muscular left ventricle contracts, the oxygen-rich blood received from the left atrium is pumped out to the rest of the body.
Source: Life Processes, Section 5.4.1
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The key sequence to remember: lungs → left atrium → left ventricle → body. The left ventricle contracts to push oxygenated blood into the aorta and out to the body. The right side handles deoxygenated blood going to the lungs — a common distractor. Examiners expect you to identify the correct chamber (ventricle, not atrium) and the correct side (left, not right).