**Core Concept**
Coronary steal syndrome refers to a condition where there is a diversion of blood flow from a normal coronary artery to a diseased or occluded coronary artery, often resulting in ischemia or infarction of the myocardium supplied by the normal artery.

**Why the Correct Answer is Right**
Coronary steal syndrome typically occurs in the presence of a significant coronary artery stenosis or occlusion, which leads to a decrease in the pressure gradient between the aorta and the distal coronary artery. This decrease in pressure gradient reduces the flow of blood through the stenotic or occluded artery, causing a compensatory increase in flow through the normal artery via collateral vessels. However, this increased flow through the normal artery can lead to a steal of oxygenated blood away from the myocardium it normally supplies, resulting in ischemia or infarction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because coronary steal syndrome is not typically associated with an increase in myocardial oxygen demand, such as during exercise or stress.
**Option B:** This option is incorrect because coronary steal syndrome is not a direct result of a coronary artery aneurysm.
**Option C:** This option is incorrect because coronary steal syndrome is not typically associated with a decrease in systemic blood pressure.

**Clinical Pearl / High-Yield Fact**
Coronary steal syndrome can be a challenging condition to diagnose and manage, as it often presents with atypical chest pain and ECG changes that can be mistaken for other coronary syndromes.

**Correct Answer: D. A significant coronary artery stenosis or occlusion**