Final answer:
The patient is likely experiencing stable angina as the chest pain was triggered by exertion and improved with rest and nitroglycerin, which aligns with the characteristics of stable angina.
Step-by-step explanation:
The patient's chest pain that occurred during lawn work and subsided after resting and taking a nitroglycerin tablet is indicative of stable angina. Stable angina is typically triggered by exertion and relieved quickly with rest or nitroglycerin, which dilates the coronary arteries and improves blood flow. The fact that the pain is improving after such interventions supports this diagnosis. Unlike stable angina, unstable angina occurs at rest, lasts longer than 15 minutes, and can be of new onset, potentially indicating an imminent heart attack. Hence, the patient does not appear to be experiencing unstable angina, Prinzmetal's angina, which is caused by coronary artery spasm, nor a myocardial infarction, which generally requires immediate treatment beyond nitroglycerin and likely would not subside so rapidly after a single dose.