While undergoing hemodialysis, the client becomes restless and tells the nurse he has a headache and feels nauseous. Which of the following complications does the nurse suspect?

In this scenario, the client undergoing hemodialysis is experiencing symptoms like restlessness, a headache, and nausea. These symptoms are indicative of an air embolus, a serious complication that can occur during hemodialysis. Air embolus happens when air enters the bloodstream and can lead to symptoms like restlessness, a headache, and nausea. It is crucial for the nurse to suspect and address this complication promptly to prevent further harm to the client. Choices A and D (Infection) are less likely in this case, as the symptoms presented are more suggestive of an air embolus rather than an infection. Choice B (Disequilibrium syndrome) is also less likely as the symptoms described are not typical of this syndrome. Therefore, the correct answer is C: Air embolus.