Iron Deficiency Anemia: This is often the most common cause of anemia in adults, resulting from chronic blood loss, inadequate iron intake, or increased iron requirement. It's particularly likely if the patient has a history of gastrointestinal bleeding, ulcers, or takes non-steroidal anti-inflammatory drugs (NSAIDs) regularly.

Anemia of Chronic Disease: This form of anemia occurs in patients with chronic infections, inflammatory conditions (like rheumatoid arthritis), or malignancies. It's a common cause of anemia and should be considered, especially if the patient has a known chronic condition.
Vitamin Deficiency Anemia (B12 or Folate Deficiency): Deficiencies in vitamin B12 or folate can lead to anemia. This could be due to dietary deficiencies, malabsorption syndromes, or increased demand. It's more likely in individuals with a restricted diet or those who have had gastrointestinal surgery.
Chronic Kidney Disease (CKD): Anemia is a common complication of CKD, resulting from decreased erythropoietin production by the kidneys. This diagnosis should be considered, especially if the patient has risk factors for CKD, such as diabetes or hypertension.

Gastrointestinal Cancer: Although less common, gastrointestinal cancers (like colon or stomach cancer) can cause anemia due to chronic blood loss. Missing this diagnosis could be fatal, so it's crucial to investigate, especially in older adults or those with a family history of cancer.
Hemoglobinopathies (e.g., Sickle Cell Disease or Thalassemia): While less common in adults presenting for the first time, these genetic disorders can cause anemia. They are critical to diagnose due to their significant impact on quality of life and the potential for serious complications.
Leukemia or Lymphoma: These hematologic malignancies can cause anemia among other symptoms. Early diagnosis is crucial for effective treatment, making them "do not miss" diagnoses.

Pernicious Anemia: An autoimmune condition leading to vitamin B12 deficiency due to the lack of intrinsic factor, necessary for B12 absorption. It's rare but important to diagnose due to its specific treatment requirements.
Paroxysmal Nocturnal Hemoglobinuria (PNH): A rare, acquired, life-threatening disease of the blood characterized by the destruction of red blood cells, bone marrow failure, and the potential for thrombotic events.
Myeloproliferative Neoplasms (e.g., Polycythemia Vera, Essential Thrombocytosis): Although these conditions more commonly present with elevated blood cell counts, some forms can lead to anemia, especially in advanced stages or with specific mutations.