**Question:** The nitrate which does not undergo first pass metabolism is -

A. Isosorbide mononitrate
B. Nitroglycerin
C. Nitroprusside
D. Nitrofurantoin

**Core Concept:** First pass liver/intestine drug absorption refers to the process where a drug is taken up by the liver or intestines before entering the systemic circulation. This can lead to reduced bioavailability and effectiveness of the drug.

**Why the Correct Answer is Right:**

Nitroglycerin (B) is a nitrate drug used for treating angina and hypertension. It undergoes rapid systemic absorption and bypasses first pass liver/intestinal absorption due to its lipophilic nature. This allows it to act directly on the blood vessels, leading to vasodilation and relief of angina symptoms.

**Why Each Wrong Option is Incorrect:**

Isosorbide mononitrate (A) is another nitrate drug used for angina treatment. However, like other nitrates, it undergoes first pass liver absorption, reducing its bioavailability and effectiveness.

Nitroprusside (C) is a nitric oxide donor used for vasodilation in critically ill patients. It is a vasodilator but does not directly answer the question as it is not a nitrate drug.

Drugs like nitrofurantoin (D) are primarily taken up by the liver for excretion, not absorption. Hence, this option is irrelevant to the question's focus on nitrate drug bioavailability.

**Clinical Pearl:**

To maximize the therapeutic benefits of nitrate drugs like nitroglycerin, it is essential to administer them sublingually or transdermally (patches), as these methods bypass first pass liver/intestinal absorption, ensuring a higher systemic bioavailability and effectiveness.