Acute bleeding or occult gastrointestinal bleeding: Given the patient's history of gastric dysplasia and the sudden drop in hemoglobin (Hb) from 110 to 43 without obvious signs of bleeding, an occult gastrointestinal bleed is highly suspect. The lack of other symptoms or abnormal lab findings (except for the severe anemia) supports this diagnosis, as the body may not have had time to compensate or show other signs of blood loss.

Bone marrow suppression or failure: The patient's low white blood cell count (WCC 3.3) and platelet count (127) suggest bone marrow involvement. Given the history of malignant thymoma, there could be metastasis affecting the bone marrow or treatment-related side effects if the patient has undergone chemotherapy or radiation.
Anemia of chronic disease: With a history of malignant thymoma and other chronic conditions (primary hypertension, pulmonary embolism, osteoporosis), anemia of chronic disease is a possibility. However, the severity of the anemia (Hb 43) is more suggestive of an acute process.
Nutritional deficiencies: Although the patient's iron levels are currently normal, other nutritional deficiencies (e.g., vitamin B12 or folate deficiency) could contribute to anemia, especially given the macrocytic indices (MCV 117.3, MCH 41.4).

Splenomegaly or hypersplenism: Although not directly indicated by the provided lab values, splenomegaly could lead to sequestration of blood cells, including red blood cells, potentially causing anemia. Given the severity of the anemia, it's crucial to consider all possible causes.
Disseminated intravascular coagulation (DIC): This condition, characterized by both widespread clotting and bleeding, can lead to severe anemia. The patient's history of malignant thymoma and pulmonary embolism increases the risk for DIC.
Paraneoplastic syndromes: Certain malignancies can cause paraneoplastic syndromes that affect the bone marrow or lead to anemia. Given the patient's history of malignant thymoma, this is a consideration.

Pure red cell aplasia: A rare condition where the bone marrow fails to produce red blood cells, which could be primary or secondary to other conditions, including malignancies.
Autoimmune hemolytic anemia: Although less likely given the lack of other symptoms or lab findings indicative of hemolysis, this condition should be considered, especially if other causes are ruled out.
Myelodysplastic syndromes: These are a group of disorders caused by poorly formed or dysfunctional blood cells, which could be a consideration given the patient's age and the presence of severe anemia and low WCC and platelet counts.