Which of the following is a classic symptom of diabetes mellitus in the elderly?

Fatigue is a classic symptom of diabetes mellitus, particularly in the elderly, due to the body's inability to effectively utilize glucose for energy. In individuals with diabetes, high blood sugar levels can lead to insufficient energy being available to the cells, resulting in persistent tiredness and a general lack of energy. This is compounded in older adults, who may already experience fatigue due to other age-related health issues or conditions.

In contrast, increased energy levels, permanent weight gain, and improved memory function do not align with the classic manifestations of diabetes. Increased energy levels would typically not be associated with diabetes; rather, individuals often report a decrease in energy. Permanent weight gain is not a direct symptom of diabetes, as the condition can lead to weight loss in some cases, especially when blood sugar levels are poorly controlled. Similarly, improved memory function is not a recognized symptom of diabetes in the elderly; in fact, cognitive decline can be a concern within this demographic. Thus, fatigue stands out as a significant and classic symptom of the condition.