Final answer:
In left-sided heart failure, the patient is most at risk of developing pulmonary edema due to increased hydrostatic pressure in the pulmonary capillaries resulting from the backing up of blood.
Step-by-step explanation:
A 54-year-old man with a myocardial infarction resulting in left-sided heart failure is most at risk of developing pulmonary edema. This condition occurs because when the left ventricle is compromised and cannot pump sufficient blood into the systemic circulation, there is a buildup of fluid backed up in the pulmonary capillaries causing increased hydrostatic pressure. This pressure results in fluid being pushed out into the lung tissues, leading to difficulty in breathing and potentially life-threatening complications.