A 34-year-old woman presents to your office with an 8-month history of recurrent, crampy lower abdominal pain, bloating, and frequent loose stools. She reports that the pain is often relieved after a bowel movement. Her symptoms are exacerbated by stress, particularly during busy periods at her demanding job as a marketing executive. She experiences 3 to 5 bowel movements daily, which she describes as loose and watery, often accompanied by a sense of urgency. She denies any nocturnal diarrhea, weight loss, fever, or blood in her stool. She has tried eliminating dairy and spicy foods on her own with minimal improvement. Her past medical history is significant for generalized anxiety disorder, for which she is not currently on medication. She has no surgical history. She does not smoke, and drinks alcohol socially on weekends. Initial laboratory studies ordered by her primary care physician are as follows:

WBC: 7,500/mm³ (4,500-11,000)

Hemoglobin: 13.5 g/dL (12.0-16.0)

Hematocrit: 40% (36-46)

Platelets: 250,000/mm³ (150,000-400,000)

Na⁺: 140 mEq/L (136-145)

K⁺: 4.1 mEq/L (3.5-5.1)

Cl⁻: 102 mEq/L (98-107)

HCO₃⁻: 25 mEq/L (22-29)

BUN: 15 mg/dL (7-20)

Creatinine: 0.8 mg/dL (0.6-1.1)

Glucose: 90 mg/dL (70-99)