Final answer:
A nurse should withhold exenatide and contact the healthcare provider immediately if a client reports abdominal pain, as it could indicate pancreatitis, a serious complication.
Step-by-step explanation:
When monitoring a client who is taking exenatide for the treatment of diabetes mellitus, a nurse should notify the healthcare provider immediately if the client reports abdominal pain. This symptom could be indicative of pancreatitis, a potentially serious complication associated with exenatide. In contrast, loss of appetite, muscle weakness, and heartburn are less severe side effects that typically do not require withholding the medication. However, it's important to note that all side effects should be reported, as they can indicate the need for dosage adjustments or a change in medication.