A 7-year-old female spayed Standard Poodle presents with a history of waxing and waning lethargy, vomiting, and anorexia.
On physical examination, she is quiet but responsive, with a heart rate of 60 bpm and weak femoral pulses.
Bloodwork reveals hyponatremia, hyperkalemia, and azotemia.
Which of the following is the most definitive diagnostic test for this dog's suspected condition?
