Name a common clinical sign of diabetes mellitus in pets.

Increased thirst and urination is a hallmark clinical sign of diabetes mellitus in pets. This occurs because high levels of glucose in the blood lead to the excess glucose being excreted through the urine, dragging water along with it due to osmosis. As a result, pets with diabetes often experience polyuria (increased urination), which leads to dehydration and subsequently prompts polydipsia (increased thirst). This classic combination of symptoms helps veterinarians diagnose diabetes mellitus in affected animals.

Other options like weight gain, lethargy, and decreased appetite may be associated with different health conditions or can occur in some diabetic pets but do not specifically characterize the disease as reliably as increased thirst and urination. Weight gain, for instance, is typically more associated with other metabolic disorders, while lethargy can arise from various factors. Decreased appetite can occur in some cases but is not as distinctive for diabetes as the increased thirst and urination.