**Core Concept**
Nitroglycerin is a vasodilator used to treat angina pectoris. It belongs to the class of nitrates, which act by releasing nitric oxide, leading to the relaxation of vascular smooth muscle and subsequent vasodilation.

**Why the Correct Answer is Right**
The longest acting nitroglycerin is isosorbide dinitrate (Isordil). It is metabolized to isosorbide mononitrate (ISMN), which has a longer half-life and provides sustained vasodilation. ISMN is responsible for the prolonged action of isosorbide dinitrate. This makes it an ideal choice for prophylactic use in angina patients.

**Why Each Wrong Option is Incorrect**
**Option A:** Glyceryl trinitrate (GTN) has a short half-life and is used sublingually or transdermally for acute angina attacks. It is not the longest acting nitroglycerin.
**Option B:** Isosorbide mononitrate (ISMN) is the active metabolite of isosorbide dinitrate, but it is not the parent compound itself.
**Option C:** Nitroglycerin ointment is a topical formulation of GTN, which has a short duration of action. It is not the longest acting nitroglycerin.

**Clinical Pearl / High-Yield Fact**
When prescribing nitrates, it's essential to consider the patient's history of angina, as tolerance to nitrates can develop with prolonged use. This can lead to reduced efficacy and the need for dose escalation.

**Correct Answer:** D. Isosorbide dinitrate