What is the most likely reason for the patient's dyspnea and cough following knee replacement surgery?

The most likely reason for the patient's dyspnea and cough following knee replacement surgery is pulmonary embolism. After undergoing major orthopedic procedures like knee replacement, patients are at an increased risk of developing venous thromboembolism, including deep vein thrombosis (DVT) that can progress to pulmonary embolism. Immobility during and after such surgeries, combined with surgical trauma, can lead to blood clots forming in the veins of the legs. If one of these clots dislodges and travels to the pulmonary arteries, it can result in a blockage, causing symptoms such as sudden onset dyspnea, cough, chest pain, and in some cases hemoptysis.

In the context of this patient post-surgery, the likelihood of developing a pulmonary embolism is higher due to the procedure's association with reduced mobility and the physiological changes following surgery. Other potential causes for dyspnea and cough could be considered, but pulmonary embolism is a critical condition that must be ruled out, especially in the post-operative setting where the risk factors are more pronounced.