**Core Concept**
Variant angina, also known as Prinzmetal's angina, is a type of angina pectoris caused by transient coronary artery spasm. The management of variant angina involves medications that prevent or relieve coronary artery spasm.

**Why the Correct Answer is Right**
Nitroglycerin is a potent vasodilator that can worsen coronary artery spasm in variant angina, exacerbating the condition. This is because nitroglycerin causes smooth muscle relaxation, but in the context of variant angina, the coronary arteries are already in a spasm. The relaxation of the smooth muscle can further reduce the diameter of the coronary arteries, leading to increased ischemia and worsening of symptoms. Therefore, nitroglycerin is contraindicated in variant angina.

**Why Each Wrong Option is Incorrect**
**Option A:** Calcium channel blockers are actually a preferred treatment for variant angina, as they help to prevent or relieve coronary artery spasm.
**Option B:** Beta-blockers are also commonly used in the management of variant angina, as they reduce myocardial oxygen demand and can help to prevent coronary artery spasm.
**Option C:** Nitrates, such as isosorbide mononitrate, are often used to prevent or relieve coronary artery spasm in variant angina.

**Clinical Pearl / High-Yield Fact**
In variant angina, the use of oxygen therapy can also be counterproductive, as it can increase the metabolic rate of the myocardium, leading to increased oxygen demand and worsening of symptoms.

**Correct Answer: D. Nitroglycerin**