A 21-year-old woman has frequent, loose stools that worsen with anxiety. What is the likely cause of her diarrhea?

The most likely cause of the 21-year-old woman's frequent loose stools that worsen with anxiety is irritable bowel syndrome (IBS). IBS is a functional gastrointestinal disorder characterized by abdominal pain, discomfort, and changes in bowel habits, which can include diarrhea, constipation, or alternating between both. The fact that her symptoms are exacerbated by anxiety suggests a strong connection often seen in IBS, where stress can trigger or amplify gastrointestinal issues.

Individuals with IBS may experience symptoms that are not attributable to any identifiable structural or biochemical abnormalities, which aligns with the nature of her symptoms. The relationship between stress and gastrointestinal function is well-documented, as emotional states can influence bowel motility and sensation.

In contrast, infectious gastroenteritis typically presents with more acute symptoms, including fever and significant abdominal cramping, and is usually linked to a recent infection or exposure to contaminated food or water. Celiac disease is an autoimmune disorder that causes an adverse reaction to gluten, leading to specific gastrointestinal symptoms, but it would not typically correlate with increased symptoms during periods of anxiety. Food intolerances can cause gastrointestinal disturbances, but they tend to be more directly linked to the consumption of specific foods rather than psychological factors like anxiety.