This senior dog now also struggles to get up on the couch. Take a look at Jack's photo, assess his initial laboratory findings, and decide what test to do next.
Jack, a 10-year-old neutered male Labrador retriever weighing 24 kg (53 lb), is presented for prolonged polydipsia and polyuria. The client has not noticed a change in Jack’s appetite or demeanor, but mentions Jack now struggles to get up on the sofa. On physical examination, Jack is bright and alert but his rear limb musculature is poor and his liver extends beyond the costal margins (see photo above). No other changes are noted on abdominal palpation. Jack is also panting throughout the examination, but auscultation of his heart and lungs is unremarkable. Fundic and rectal examinations are also unremarkable.
A complete blood count shows a stress leukogram with a hematocrit of 49% and adequate platelets. The abnormal results of a full serum chemistry profile include hypercholesterolemia (396 mg/dl; reference range = 120-247 mg/dl), elevated alanine transaminase activity (197 U/L; reference range = 10-130 U/L), and elevated alkaline phosphatase activity (782 U/L; reference range = 24-147 U/L). Urine specific gravity is 1.006, pH is 6.5, trace protein is found, and the urine sediment examination is unremarkable. A urine culture is negative.
Results of an adrenocorticotrophic hormone (ACTH) stimulation test using 5 μg/kg intramuscularly of cosyntropin are as follows: both baseline and post-ACTH-administration cortisol concentrations are within normal limits at 3.4 µg/dl and 12.7 µg g/dl, respectively.
Which of the following diagnostic choices is most appropriate to perform next for this patient?