Which of the following is a common symptom of an atrial myxoma?

Shortness of breath is a common symptom of an atrial myxoma due to the mass potentially obstructing blood flow in the heart, which can lead to compromised cardiac output. As the myxoma grows, it can interfere with the heart's ability to effectively pump blood, particularly if it is located in the left atrium. This obstruction can result in pulmonary congestion, hence causing the feeling of breathlessness.

While weight gain, chest pain, and a persistent cough can also occur due to various cardiovascular issues, they are not as directly connected to the presence of an atrial myxoma. Weight gain may happen as a result of fluid retention from heart failure, chest pain could arise from ischemic heart disease, and a persistent cough might indicate other respiratory or cardiac conditions. However, shortness of breath remains the most characteristic and immediate symptom associated with the physical presence of the tumor in the heart, making it the best choice.