Which of the following treatments is indicated for myocardial infarction?

Sublingual nitroglycerin is an appropriate treatment for myocardial infarction because it acts as a rapid vasodilator. This medication helps to widen the blood vessels, improving blood flow to the heart muscle, which is crucial during a myocardial infarction when the heart is in distress. The goal of administering nitroglycerin is to relieve chest pain and reduce the workload on the heart, ultimately minimizing damage to the heart tissue.

In contrast, increased fluid intake and dietary changes do not address the immediate needs of a patient experiencing a myocardial infarction. These options may be relevant in broader cardiac health management but are not specific interventions during an acute event. Physical therapy, while beneficial for rehabilitation after a myocardial infarction, is not indicated during the acute phase of the event itself; immediate treatment focuses on restoring blood flow and alleviating symptoms, making sublingual nitroglycerin the correct choice.