Final answer:
The most likely cause of excess fluid in a patient's lungs is increased hydrostatic pressure in the pulmonary capillaries. This condition often occurs due to heart failure resulting in pulmonary edema.
Step-by-step explanation:
Excess fluid in the patient's lungs is most likely caused by increased hydrostatic pressure at the pulmonary sites of gas exchange. In conditions like heart failure, when the left ventricle cannot pump enough blood into the systemic circulation, the blood backs up in the pulmonary circulation. This results in increased hydrostatic pressure in the pulmonary capillaries, which pushes fluid out into the lung tissue, leading to pulmonary edema.