Which of the following indicates a heart's adaptation due to increased physical demands?

Myocardial hypertrophy is a physiological response of the heart that occurs when it is subjected to increased physical demands, such as during intense exercise or from conditions that require the heart to work harder, like hypertension. This adaptation involves the increase in the size of the heart muscle, leading to a greater capacity to pump blood effectively.

In athletes, for example, the heart adapts by enlarging its walls, thus enhancing overall cardiac output and performance without necessarily indicating pathological changes. Such adaptation can be beneficial, allowing the heart to meet the body's increased requirements for oxygenated blood during physical activity.

In contrast, other options, such as heart failure, denote a pathological condition where the heart cannot pump effectively, and cardiac stress can refer broadly to various factors that place stress on the heart but do not specifically indicate a positive adaptation. Myocardial infarction, or heart attack, is caused by a blockage of blood flow and is a critical, acute condition rather than an adaptive process. Thus, myocardial hypertrophy accurately reflects the heart's ability to adapt positively to increased physical demands.