Hypertension: This is often considered the most common systemic cause of epistaxis. Elevated blood pressure can lead to the rupture of small blood vessels in the nose, resulting in bleeding. The justification for this being the single most likely diagnosis is based on the prevalence of hypertension in the population and its well-documented association with epistaxis.

Anticoagulation Therapy: Patients on anticoagulants (such as warfarin, aspirin, or novel oral anticoagulants) are at an increased risk of epistaxis due to the medication's effect on blood clotting. This is a common and well-recognized cause of epistaxis in patients taking these medications.
Vitamin K Deficiency: A deficiency in vitamin K can impair the production of clotting factors, leading to an increased risk of bleeding, including epistaxis. This condition, though less common, is a plausible cause in patients with malnutrition or those taking certain medications that interfere with vitamin K metabolism.
Liver Disease: Liver dysfunction can lead to coagulopathy due to decreased production of clotting factors, which in turn can cause epistaxis. The liver's role in producing these factors makes liver disease a likely cause of bleeding disorders, including epistaxis.

Bleeding Disorders (e.g., Hemophilia, von Willebrand Disease): Although less common, these conditions can lead to severe and recurrent epistaxis. Missing these diagnoses could result in inadequate management and significant morbidity.
Vasculitis (e.g., Granulomatosis with Polyangiitis): Certain types of vasculitis can cause epistaxis as part of their presentation. These conditions are potentially life-threatening and require prompt diagnosis and treatment.
Tumor (e.g., Nasopharyngeal Carcinoma): Although rare, tumors in the nasal cavity or sinuses can cause epistaxis. The potential for malignancy makes this a critical diagnosis not to miss.

Hereditary Hemorrhagic Telangiectasia (Osler-Weber-Rendu Syndrome): This is a rare genetic disorder characterized by the formation of abnormal blood vessels, which can lead to epistaxis among other symptoms.
Pseudoxanthoma Elasticum: A rare genetic disorder that affects the elastic fibers present in some tissues, including blood vessels, potentially leading to bleeding, including epistaxis.
Scurvy: Caused by vitamin C deficiency, scurvy can lead to poor collagen synthesis, affecting blood vessel integrity and potentially causing epistaxis. This condition is rare in developed countries but can occur in cases of severe malnutrition.