Recurrent Cyclic Vomiting and Gastroparesis-Like Symptoms in a Patient With Mast Cell Activation Syndrome (MCAS): A Case of Gastrointestinal-Predominant MCAS.

Mast cell activation syndrome (MCAS) remains an underrecognized cause of gastrointestinal morbidity, frequently presenting with symptoms indistinguishable from functional disorders. Such diagnostic ambiguity can misdirect management and prolong patient symptoms. Thus, we present a case of a 36-year-old male with MCAS who presented with acute, intractable nausea, vomiting, and left-sided back pain. Examination revealed tachycardia and tenderness in the left costovertebral angle. Laboratory evaluation revealed mild anemia, thrombocytopenia, and slightly elevated bilirubin levels. Abdominal and pelvic CT was unremarkable, and upper endoscopy showed mild mucosal erythema with retained gastric contents. Gastric emptying scintigraphy confirmed delayed gastric emptying consistent with mast cell-mediated dysmotility. This case highlights an uncommon gastrointestinal presentation of mast cell activation, emphasizing the importance of considering MCAS in patients with recurrent unexplained vomiting and delayed gastric emptying.