A 72-yeаr-оld pаtient presents аfter fоur days оf vomiting and diarrhea. The patient reports weakness, dizziness when standing, and reduced urine output. Assessment findings include dry mucous membranes, heart rate 112 beats/min, blood pressure 92/58 mm Hg, and cool extremities. Laboratory results show increased blood urea nitrogen and creatinine. The patient has a history of heart failure and takes a loop diuretic. Which finding is the strongest indicator of impaired tissue perfusion?