**Core Concept**
The experimental drug is targeting the regulation of coronary blood flow, specifically affecting the autoregulatory response of coronary arterioles. Autoregulation is a vital mechanism that maintains a relatively constant blood flow to the myocardium despite fluctuations in systemic blood pressure. This is achieved through the regulation of small arterioles, which are sensitive to changes in oxygen demand and metabolic byproducts.

**Why the Correct Answer is Right**
The drug's action on autoregulatory coronary arterioles is likely mediated through the endothelium-derived relaxing factor (EDRF), now known as nitric oxide (NO). NO is produced by the endothelial cells lining the arterioles and causes relaxation of vascular smooth muscle cells, leading to vasodilation. The preferential effect on small arterioles suggests that the drug is not acting on the major conducting vessels, which are influenced by other mechanisms such as sympathetic tone and local metabolic factors.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is not provided.
**Option B:** This choice is not provided.
**Option C:** This choice is not provided.
**Option D:** This choice is not provided.

**Clinical Pearl / High-Yield Fact**
The concept of autoregulation is crucial in understanding the regulation of coronary blood flow. It's essential to remember that the autoregulatory response is primarily mediated through the small arterioles, which are sensitive to changes in metabolic demand and oxygen supply. This knowledge is vital in understanding the pathophysiology of coronary artery disease and the effects of various interventions on coronary blood flow.

**Correct Answer:** D.