A 58-Year-Old Man Is Brought to the Physician by His

A 58-year-old man is brought to the physician by his wife because of a 10-day history of progressively worsening diarrhea and bloating. He has been having 8-10 episodes of loose, watery stools each day, including during the night. He does not have nausea, vomiting, or blood in his stool. He has a history of metastatic colon cancer and started chemotherapy 2 weeks ago. He also has Wolff-Parkinson-White syndrome, for which he takes procainamide. He smoked one pack of cigarettes daily for 25 years and drank 2-3 beers daily but quit both last year when his grandson was born. His temperature is 37.3°C (99.1°F) , pulse is 114/min, respirations are 22/min, and blood pressure is 105/68 mm Hg. Physical examination shows pale skin with a decreased turgor; capillary refill time is 2 seconds. His abdomen is distended and tender on palpation; bowel sounds are hyperactive. Laboratory studies show:
Hematocrit 55%
Leukocyte count 18,000/mm3
Serum
Na+ 149 mEq/L
K+ 3.3 mEq/L
Cl- 117 mEq/L
Magnesium 1.8 mg/dL
HCO₃- 20 mEq/L
Glucose 78 mg/dL
Creatinine 1.0 mg/dL
Blood and stool cultures are negative. A test for Clostridioides difficile toxins is negative. In addition to oral rehydration, which of the following is the most appropriate initial step in management?

A) Rectal budesonide
B) Oral rifaximin
C) Oral vancomycin
D) Intravenous octreotide
E) Oral loperamide