A 72-year-old man with diabetic kidney disease takes perindopril and spironolactone. He has a serum potassium concentration of 6.2 mmol/L (3.5–5.2), chloride 115 mmol/L (95–110), bicarbonate 16 mmol/L (22–32), glucose 9.8 mmol/L (3.5–7.7) and a normal lactate concentration. Urine pH is 5.0, and testing for ketones is negative. Which one of the following is the most likely explanation for these findings?