Final answer:
Patients in atrial fibrillation for more than 48 hours or when time is uncertain require a Transesophageal Echocardiogram (TEE) or anticoagulation before undergoing cardioversion to decrease the risk of blood clots.
Step-by-step explanation:
Cardioversion is a medical procedure to restore a normal heart rhythm in patients with certain types of abnormal heartbeats, such as atrial fibrillation. According to current guidelines, patients who have been in atrial fibrillation for more than 48 hours, or when the duration is unknown, should undergo a Transesophageal Echocardiogram (TEE) or receive adequate anticoagulation prior to the procedure to reduce the risk of thromboembolic events. This means the correct answer to the student's question is (b) 48 hours. The TEE is used to exclude the presence of atrial thrombi, while anticoagulation therapy minimizes the risk of clot formation due to the irregular heart rhythms associated with atrial fibrillation.