Agent of first choice in an acute attack of Prinzmetal&;s angina is -

Ref : harrisons-manual-of-medicine-16th-edition pg no: 637 PRINZMETAL'S VARIANT ANGINA (CORONARY VASOSPASM) Intermittent focal spasm of coronary aery; often associated with atherosclerotic lesion near site of spasm. Chest discomfo is similar to angina but more severe and occurs typically at rest, with transient ST-segment elevation. Acute infarction or malignant arrhythmias may develop during spasm-induced ischemia. Evaluation includes observation of ECG (or ambulatory Holter monitor) for transient ST elevation; diagnosis confirmed at coronary angiography using provocative (e.g., IV acetylcholine) testing. Primary treatment consists of longacting nitrates and calcium antagonists. Prognosis is better in pts with anatomically normal coronary aeries than those with fixed coronary stenoses.