Review Question - QID 212826

A 55-year-old man presents to the emergency department with fatigue and a change in his memory. The patient and his wife state that over the past several weeks the patient has been more confused and irritable and has had trouble focusing. He has had generalized and non-specific pain in his muscles and joints and is constipated. His temperature is 99.3°F (37.4°C), blood pressure is 172/99 mmHg, pulse is 79/min, respirations are 14/min, and oxygen saturation is 99% on room air. Physical exam is unremarkable. Laboratory studies are ordered as seen below.

Hemoglobin: 9.0 g/dL
Hematocrit: 30%
Leukocyte count: 6,500/mm^3 with normal differential
Platelet count: 166,000/mm^3
MCV: 78 fL

Serum:
Na+: 141 mEq/L
Cl-: 103 mEq/L
K+: 4.6 mEq/L
HCO3-: 25 mEq/L
BUN: 20 mg/dL
Glucose: 99 mg/dL
Creatinine: 0.9 mg/dL
Ca2+: 10.2 mg/dL

Which of the following is the most likely diagnosis?