Which of the following is a cause of tension in a reversible cardiac arrest?

Cardiac tamponade is a condition characterized by the accumulation of fluid in the pericardial space, leading to increased pressure on the heart. This pressure inhibits the heart's ability to fill properly during diastole, which can severely impair cardiac output and lead to a state of shock or, in extreme cases, cardiac arrest. In reversible cardiac arrest situations, addressing the underlying cause—such as relieving the pressure through pericardiocentesis—can restore normal heart function.

The other conditions listed can lead to cardiac arrest but typically do not fall under the category of tension-related issues where immediate pressure relief can cause a dramatic improvement. For instance, thrombosis refers to blood clots that can obstruct flow but do not involve the pressure dynamics within the heart and surrounding structures like cardiac tamponade does. Hyperkalemia involves elevated potassium levels, which can disrupt cardiac electrical activity rather than creating external pressure. Acidosis reflects a disturbance in acid-base balance, which can also lead to cardiac issues but is not related to pressure within the heart's surroundings. Hence, cardiac tamponade is identified specifically for its reversible nature when the tension (fluid pressure) is alleviated.