A 47-year-old woman comes to the physician with a 3-week history of a dry, nonproductive cough. She does not smoke and denies illicit drug use. Physical examination shows mild conjunctival injection, and lung auscultation reveals fine crackles bilaterally. Laboratory testing demonstrates a total serum calcium concentration of 10.8 mg/dL. Spirometry shows a restrictive ventilatory pattern compared with predicted values. Further evaluation of this patient is most likely to show an increase in which of the following?