A 53-year-old man with multiple myeloma is hospitalized after 2 days of polyuria, polydipsia, and increasing confusion. Laboratory tests show an elevated serum $\left[\mathrm{Ca}^{2+}\right]$ of $15 \mathrm{mg} / \mathrm{dL}$, and treatment is initiated to decrease it. The patient's serum osmolarity is $310 \mathrm{mOsm} / \mathrm{L}$.
-The most likely reason for polyuria in this man is

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[Chapter 5, VII D 3; Table 5.6]. The most likely explanation for this patient's polyuria is hypercalcemia. With severe hypercalcemia, $\mathrm{Ca}^{2+}$ accumulates in the inner medulla and papilla of the kidney and inhibits adenylate cyclase, blocking the effect of $\mathrm{ADH}$ on water permeability. Because ADH is ineffective, the urine cannot be concentrated and the patient excretes large volumes of dilute urine. His polydipsia is secondary to his polyuria and is caused by the increased serum osmolarity. Psychogenic water drinking would also cause polyuria, but the serum osmolarity would be lower than normal, not higher than normal.

A 53-Year-Old Man with Multiple Myeloma Is Hospitalized After 2 [Ca2+]\left[\mathrm{Ca}^{2+}\right][Ca2+]

A 53-Year-Old Man with Multiple Myeloma Is Hospitalized After 2 $\left[\mathrm{Ca}^{2+}\right]$