Excess Vitamin D

A 29-year-old woman is brought to the emergency department by her partner for evaluation of progressive confusion. She also complains of nausea and vomiting for the past 24-hours. Past medical history is significant for asthma and a chronic cough for the past year. The patient has had 2 episodes of uveitis in the past 6 months treated with topical therapy. Current medications include vitamin supplementation and inhaled albuterol as needed. Family history is noncontributory. She does not use tobacco, alcohol, or illicit drugs. Temperature is 37.2 C (98.9 F), pulse is 98/min, respirations are 18/min and blood pressure is 100/60 mmHg. Physical examination shows dry mucous membranes and multiple tender pink-to-reddish nodules below the knee. Chest x-ray shows bilateral hilar lymphadenopathy. Laboratory studies are performed and shown below:  
 

Laboratory value	Result
Serum chemistry
Sodium	132 mEq/L
Potassium	4.2 mEq/L
Chloride	95 mEq/L
Calcium	11.8 mg/dL
Creatinine	0.8 mg/dL
Blood urea nitrogen	20 mg/dL

Serum ACE levels are increased. HIV testing is negative. Which of the following serum substance levels is most likely to be increased in this patient?