A Mallory-Weiss tear is a longitudinal mucosal laceration at the gastroesophageal junction (GEJ) or gastric cardia caused by sudden increases in intra-abdominal pressure against a closed glottis, most commonly from forceful retching or vomiting. The pathophysiology involves acute mechanical stress on the GEJ mucosa: as intra-abdominal pressure rises dramatically during retching, the gastric fundus herniates through the diaphragmatic hiatus, stretching the mucosal junction beyond its tensile strength. The tear typically involves the mucosa and submucosa without penetrating the muscularis propria (distinguishing it from Boerhaave syndrome, which is full-thickness esophageal rupture). Risk factors include alcoholism, bulimia, violent vomiting from any cause, hiatal hernia, and retching during endoscopy. The tear disrupts submucosal arteries, producing hematemesis that is typically self-limited (90% stop spontaneously) but can be massive if a large artery is involved. The nurse monitors vital signs for hemodynamic instability, assesses hematemesis volume and character, obtains IV access and type-and-screen, monitors hemoglobin, administers IV proton pump inhibitors, provides antiemetic therapy to prevent further retching, and assists with endoscopic intervention (injection, cauterization, or clipping) for ongoing hemorrhage.