Upper Gastrointestinal Bleed (UGIB): The patient's presentation with dark red emesis (vomiting blood) and being on apixaban, an anticoagulant, makes UGIB the most likely diagnosis. The use of anticoagulants increases the risk of bleeding, and UGIB is a common cause of hematemesis (vomiting blood) in such patients.

Gastroesophageal Variceal Bleed: Although less likely than a non-variceal UGIB given the lack of liver disease mentioned, it's still a consideration, especially if the patient has underlying liver issues not yet identified.
Mallory-Weiss Tear: This could be a possibility if the patient had an episode of forceful vomiting before the onset of hematemesis, but it's less likely given the description of dark red emesis with clots, suggesting a more significant bleed.
Erosive Gastritis or Ulcer Disease: These conditions can cause UGIB and are more likely in the context of anticoagulation therapy.

Aortic Dissection: Although the patient's symptoms do not classically suggest aortic dissection (e.g., severe, tearing chest pain), the presence of hypotension and a history of hypertension (implied by chronic atrial fibrillation and non-ischemic cardiomyopathy) makes it crucial not to miss this potentially fatal diagnosis.
Pulmonary Embolism: Given the patient's history of chronic atrial fibrillation and non-ischemic cardiomyopathy, along with symptoms of fatigue, it's essential to consider pulmonary embolism, especially if there are signs of right heart strain or if the patient reports dyspnea.
Cardiac Tamponade: The patient's hypotension and history of cardiomyopathy raise the possibility of cardiac tamponade, which would require urgent diagnosis and treatment.

Hemobilia: Bleeding from the biliary tract into the gastrointestinal tract, which could be considered if there's a history of liver or biliary disease, but it's rare and less likely without specific risk factors.
Dieulafoy's Lesion: A rare cause of UGIB characterized by a small, superficial ulceration with an exposed, often large, caliber artery at its base, which can cause significant bleeding.
Gastrointestinal Angiodysplasia: Although more common in the elderly, this condition is less likely to present acutely without a history of previous bleeding episodes or known vascular ectasias.