A 75-year-old male shows disorientation and incoherence after surgery. What is the key factor behind his symptoms?

The symptoms of disorientation and incoherence in a 75-year-old male after surgery are most closely associated with delirium. Delirium is an acute confusional state that often occurs in older adults, particularly following surgical procedures. Various factors contribute to the development of delirium, including changes in medication, metabolic imbalances, pain, dehydration, and the effects of anesthesia.

In elderly patients, the experience of the stress of surgery combined with potential medication side effects can exacerbate cognitive dysfunction, leading to delirium. Symptoms of delirium can manifest quickly, with disorientation and incoherence being common presentations. The acute nature of these symptoms, particularly following surgery, aligns with what is typical in cases of delirium.

While infection, medication side effects, and stroke are potential causes for altered mental status, they are less immediately characteristic of the post-operative context where delirium is frequently observed in older adults. Therefore, the most relevant explanation for the symptoms described is delirium, making it the key factor behind the patient's condition.