In a senior dog with hypercalcemia and isosthenuria, search for which condition would be most appropriate?

• A. Hypercalcemia of malignancy
• B. Primary hyperparathyroidism
• C. Renal failure due to dehydration
• D. Hypocalcemia

Answer: (A)

When calcium is high and the kidneys are failing to concentrate urine (isosthenuria), think about a cancer-related cause that releases factors driving calcium up and impairing renal concentrating ability. In older dogs, malignancy is a common source of marked hypercalcemia because tumors can secrete PTH-related peptide, which raises calcium, and the resulting hypercalcemia can cause nephrogenic diabetes insipidus–like water loss, leading to isosthenuria.

So the most appropriate condition to look for is hypercalcemia of malignancy, such as cancers that commonly produce PTHrP (for example, anal sac adenocarcinoma or certain lymphomas). This ties the hypercalcemia with the reduced urine concentration. The other options don’t fit as well: primary hyperparathyroidism can cause hypercalcemia but isn’t as strongly linked to isosthenuria; dehydration-related renal failure explains poor concentrating ability but not the hypercalcemia itself; hypocalcemia is the opposite of what’s described.