**Core Concept**
Nitroglycerine is a vasodilatory medication used in the management of angina pectoris. Nitroglycerine preparations vary in their duration of action, which is crucial for clinical efficacy and patient compliance.

**Why the Correct Answer is Right**
The longest acting nitroglycerine preparation is isosorbide mononitrate (ISMN). ISMN is a long-acting nitrate that is metabolized to its active form, which then slowly releases nitric oxide, causing sustained vasodilation. This leads to a prolonged duration of action, typically lasting up to 10-12 hours. ISMN's mechanism of action involves the inhibition of smooth muscle tone in blood vessels, resulting in decreased peripheral resistance and reduced myocardial oxygen demand.

**Why Each Wrong Option is Incorrect**
* **Option A:** Nitroglycerine ointment has a relatively short duration of action, typically lasting around 20-30 minutes, due to rapid absorption and metabolism.
* **Option B:** Isosorbide dinitrate (ISDN) is a short-acting nitrate with a duration of action similar to nitroglycerine ointment, usually lasting around 30-60 minutes.
* **Option D:** Glyceryl trinitrate (GTN) tablets or sprays have a short duration of action, typically lasting around 30-60 minutes, due to rapid absorption and metabolism.

**Clinical Pearl / High-Yield Fact**
When selecting a nitrate preparation, consider the patient's specific needs and clinical scenario. Long-acting nitrates like ISMN are ideal for nocturnal angina or patients with severe angina, while short-acting nitrates like GTN are better suited for acute angina attacks or peri-procedural pain management.

**Correct Answer: C. Isosorbide mononitrate (ISMN)**