What clinical finding is most likely indicative of a urinary tract infection in dogs?

Hematuria, or the presence of blood in the urine, is a clinical finding that strongly suggests a urinary tract infection (UTI) in dogs. This symptom occurs because inflammation and irritation of the urinary tract lining, which can result from an infection, may lead to damage of blood vessels and the subsequent leaking of blood into the urine. In addition to hematuria, other common signs of a UTI may include increased frequency of urination, straining to urinate, and inappropriate urination behaviors.

In contrast, the other options do not specifically point to a urinary tract infection. Weight loss may be a sign of many chronic conditions but is not directly indicative of UTIs. Diarrhea is more commonly associated with gastrointestinal issues rather than urinary tract problems. Excessive grooming could indicate various skin conditions or discomfort but is not a typical response associated with UTIs. Therefore, hematuria is the most relevant and clear indicator of a urinary tract infection in dogs.