What is a common clinical symptom of diabetes mellitus?

Increased thirst and frequent urination are hallmark clinical symptoms of diabetes mellitus. This occurs due to elevated blood glucose levels, which lead to osmotic diuresis. When the body's glucose levels exceed the renal threshold, glucose spills into the urine, pulling water with it and resulting in increased urine output. The loss of fluid leads to dehydration, which in turn causes increased thirst, as the body attempts to compensate for the fluid loss.

The other options, while they may be associated with diabetes to some extent, do not specifically capture the classic symptoms as effectively as increased thirst and frequent urination. Weight loss and high blood pressure are not definitive symptoms of diabetes; weight loss can occur in uncontrolled diabetes but is not a common symptom in all patients, and high blood pressure is more of a related condition rather than a direct symptom of diabetes itself. Fatigue and blurred vision can occur, particularly if blood glucose levels are not well managed, but these symptoms are less common compared to the more direct indicators of thirst and urination. Allergic reactions and skin rashes are generally not considered symptoms of diabetes mellitus and are more indicative of other medical issues or conditions.