Review Question - QID 212675

A 57-year-old man presents to the emergency department with confusion. His symptoms started a few days ago and have been gradually worsening. Initially, the patient had a fever and a cough which has progressed to abdominal pain, diarrhea, and confusion. His temperature is 102°F (38.9°C), blood pressure is 127/68 mmHg, pulse is 120/min, respirations are 17/min, and oxygen saturation is 94% on room air. Physical exam is notable for localized crackles and decreased breath sounds in the left lower lung field. The patient is confused and is answering questions inappropriately. Laboratory studies are ordered as seen below.

Hemoglobin: 12 g/dL
Hematocrit: 36%
Leukocyte count: 16,500/mm^3 with normal differential
Platelet count: 190,000/mm^3

Serum:
Na+: 130 mEq/L
Cl-: 100 mEq/L
K+: 4.3 mEq/L
HCO3-: 25 mEq/L
BUN: 20 mg/dL
Glucose: 99 mg/dL
Creatinine: 1.1 mg/dL
Ca2+: 10.2 mg/dL

Which of the following is the best diagnostic test to guide current therapy in this patient?