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?
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A
Endogenous ACTH concentration
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B
Low-dose dexamethasone suppression test
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C
Abdominal ultrasound
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D
ACTH stimulation test