A 28-year-old woman presents to her primary care physician with a 4-month history of bothersome gastrointestinal symptoms. She describes daily liquid, nonbloody bowel movements, often accompanied by lower abdominal cramping and persistent bloating. She notes that her symptoms are less severe in the mornings before she eats and do not disturb her sleep. She denies any significant weight loss, recent foreign travel, or antibiotic exposure. Her medical history is otherwise unremarkable. Physical examination reveals normal vital signs, and her abdominal examination is soft, non-tender, and non-distended, with normoactive bowel sounds. Considering these findings, what is the most likely diagnosis and the most appropriate initial management strategy?

A 68-year-old woman presents to the emergency department with a 4-month history of persistent, large-volume, watery diarrhea, occurring frequently throughout the day and even during fasting. She denies any significant abdominal pain, fever, or blood in her stools, but reports significant fatigue and an unintentional weight loss of 5 kg over this period. Her medical history is unremarkable, and she has no recent travel history or antibiotic use. On admission, her vital signs are: blood pressure 90/60 mm Hg, heart rate 105/min, respiratory rate 18/min, and temperature 37.2°C. Physical examination reveals dry mucous membranes, reduced skin turgor, and mild peripheral edema. Given her presentation and initial laboratory findings, what is the most likely diagnosis and the immediate next step in her management?

A 32-year-old woman presents to her primary care provider's office complaining of recurrent abdominal discomfort and loose stools. She reports that for several months, she has noticed these symptoms, including significant bloating and increased gas, occurring roughly an hour or two after consuming milk, cheese, or ice cream. She specifically recalls a recent episode where severe cramping and watery diarrhea followed a large serving of creamy pasta sauce. She denies any fever, bloody stools, or unintended weight loss. Her physical examination reveals mild generalized abdominal distension but is otherwise unremarkable, with no localized tenderness. Vital signs are stable: temperature 36.80C, blood pressure 120/78 mmHg, pulse 75/min, respirations 14/min, and oxygen saturation 99% on room air. Based on her clinical presentation, what is the most likely diagnosis and the most appropriate initial management strategy?