A 22-year-old graduate student is most likely suffering from which condition given her symptoms of abdominal pain and changing bowel habits?

The symptoms described for the 22-year-old graduate student—abdominal pain and changing bowel habits—are characteristic of Irritable Bowel Syndrome (IBS). IBS is a functional gastrointestinal disorder that commonly presents with a combination of symptoms, including abdominal pain, bloating, and alterations in bowel habits, such as diarrhea and constipation.

In young adults, particularly women, IBS is one of the most prevalent disorders linked to gastrointestinal symptoms. The variability in bowel habits indicated in the question aligns well with the hallmark presentations of IBS, which often correlates with stress, anxiety, or lifestyle changes.

While Crohn's disease and Celiac disease may also present with abdominal symptoms, they would typically include more severe manifestations such as weight loss, severe diarrhea, or nutritional deficiencies. Thyroid storm is less relevant in this context, as it would present with systemic symptoms beyond gastrointestinal concerns, such as hyperthermia, tachycardia, and altered mental status. Thus, the combination of abdominal pain and changing bowel habits in a young individual strongly supports a diagnosis of Irritable Bowel Syndrome.