Which of the following conditions is indicated by ST-segment depression during exercise?

ST-segment depression during exercise is a significant finding often associated with myocardial ischemia. When the heart muscle does not receive enough oxygen-rich blood, typically due to narrowed coronary arteries, the electrical activity of the heart is affected. During exercise, the demand for oxygen increases, and if the supply cannot meet this demand because of ischemia, the ST segment on an electrocardiogram may show depression. This is a clear indicator that the heart is under stress and can signal potential underlying coronary artery disease or other cardiac conditions that impede the heart's ability to function effectively under increased workload.

The other conditions mentioned do not typically present with ST-segment depression during exercise: pulmonary embolism may cause different signs such as a sudden onset of dyspnea or chest pain, aortic regurgitation is primarily associated with symptoms related to heart failure and a characteristic murmur, and a heart murmur itself is simply a sound made by blood flow turbulence and is not a direct indicator of electrical activity changes seen in an ECG during exercise.