Final answer:
Cardiac tamponade causes distended neck veins due to the pressure exerted on the heart and the reduced blood volume in the chambers.
Step-by-step explanation:
Cardiac tamponade causes distended neck veins because the excess fluid in the pericardial cavity puts pressure on the heart and prevents it from fully relaxing. This pressure leads to decreased blood volume in the heart chambers, resulting in less blood being ejected from the heart. The increased pressure in the heart and the reduced blood volume cause the neck veins to become distended.