What are the most likely causes of azotemia in cats and how do I differentiate pre-renal from renal from post-renal?

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Azotemia in cats is classified as prerenal, renal, or postrenal based on the origin of nitrogenous compound accumulation, and differentiating these categories requires integrating history, urinalysis, imaging, and biochemical data. 3 Prerenal azotemia results from reduced renal perfusion without intrinsic kidney damage. Causes include dehydration, hypovolemia, hypotension, and decreased cardiac output — including that driven by congestive heart failure (CHF) and its treatment with loop diuretics such as furosemide. 410 A biochemical clue is a disproportionate blood urea nitrogen (BUN) rise relative to creatinine, which reflects increased tubular urea resorption under conditions of reduced renal tubular flow and antidiuretic hormone stimulation. 8 Prerenal azotemia is expected to resolve with correction of the underlying perfusion deficit; persistent azotemia after fluid resuscitation indicates a renal or mixed component. 310 Renal azotemia reflects intrinsic parenchymal disease and encompasses acute kidney injury (AKI), chronic kidney disease (CKD), and acute-on-CKD. Serum amyloid A (SAA) concentration increases in 44% of cats with renal azotemia but does not differentiate AKI from CKD or acute-on-CKD, and does not carry prognostic value. 1 Renal ultrasonography is the most informative non-invasive tool for characterizing intrinsic disease. The ultrasound findings most strongly associated with azotemia in cats (defined as serum creatinine >180 μmol/L) are perinephric fluid, small kidneys, hyperechoic renal cortex, loss of corticomedullary differentiation, renal calculi, enlarged kidneys, and dilated renal pelvis. 5 In non-azotemic cats, mean renal length is 40.1 ± 5.5 mm; male cats have kidneys 5.2 mm longer on average than females, and the right kidney averages 1.6 mm larger than the left. 5 Renal pelvic diameter is negatively correlated with urine specific gravity, making it a useful indirect marker of concentrating ability. 5 Subjective radiologist assessment of