**Core Concept**
Ischemia during a treadmill test can manifest as alterations in the electrocardiogram (ECG), reflecting changes in myocardial oxygen supply and demand. The ECG signs of ischemia are crucial for diagnosing coronary artery disease.

**Why the Correct Answer is Right**
The correct ECG signs of ischemia during a treadmill test include ST-segment depression, T-wave inversion, and Q-wave formation. These changes occur due to the imbalance between myocardial oxygen supply and demand, leading to the activation of the ischemic cascade. ST-segment depression is the most common ECG sign of ischemia, resulting from the decreased repolarization of the ischemic myocardium. T-wave inversion is another indicator of ischemia, reflecting the altered repolarization pattern of the affected myocardial cells. Q-wave formation, although not as common, is a sign of transmural myocardial infarction.

**Why Each Wrong Option is Incorrect**
**Option A:** ST-segment elevation is not a sign of ischemia in a treadmill test; it is more commonly associated with myocardial infarction.
**Option B:** P-wave inversion is not a sign of ischemia; it is related to atrial abnormalities, such as atrial fibrillation or P mitrale.
**Option C:** QRS complex widening is not a direct sign of ischemia; it can be associated with various conditions, including bundle branch blocks or ventricular hypertrophy.

**Clinical Pearl / High-Yield Fact**
Remember that the ECG signs of ischemia in a treadmill test are dynamic and can change with exercise intensity and duration. A careful analysis of the ECG changes during and after exercise is essential for diagnosing coronary artery disease.

**Correct Answer:** C. Q-wave formation