Idiopathic Thrombocytopenic Purpura (ITP): A common cause of isolated thrombocytopenia, characterized by autoimmune destruction of platelets. The patient's long-standing low platelet count without other symptoms or medications that could cause thrombocytopenia makes ITP a plausible diagnosis.

Chronic Liver Disease: Liver diseases such as cirrhosis can lead to thrombocytopenia due to splenic sequestration. The spleen enlarges and holds onto more platelets, reducing the circulating platelet count.
Viral Infections: Certain viral infections, like HIV or hepatitis C, can cause thrombocytopenia through various mechanisms, including immune-mediated destruction of platelets.
Autoimmune Disorders: Conditions like systemic lupus erythematosus (SLE) can lead to thrombocytopenia due to the production of autoantibodies against platelets.

Leukemia or Lymphoma: Malignancies affecting the bone marrow can lead to thrombocytopenia. Although less common, missing a diagnosis of leukemia or lymphoma could have severe consequences.
Splenomegaly due to Other Causes: Conditions like lymphoma, sarcoidosis, or portal vein thrombosis can cause splenomegaly, leading to sequestration of platelets and subsequent thrombocytopenia.
Thrombotic Thrombocytopenic Purpura (TTP) or Hemolytic Uremic Syndrome (HUS): Although rare, these conditions are life-threatening and require immediate attention. They are characterized by thrombocytopenia, microangiopathic hemolytic anemia, and renal failure.

Congenital Thrombocytopenias: Rare genetic disorders that affect platelet production or function, such as Bernard-Soulier syndrome or Wiskott-Aldrich syndrome.
Drug-Induced Thrombocytopenia: Although the patient is not on blood thinners, certain medications can induce thrombocytopenia through immune-mediated mechanisms.
Inherited Bone Marrow Failure Syndromes: Conditions like Fanconi anemia or dyskeratosis congenita can lead to thrombocytopenia due to bone marrow failure.