Vitamin B12 or Folate Deficiency: This is the most common cause of macrocytosis, especially in adolescents. A deficiency in either vitamin B12 or folate can lead to impaired DNA synthesis, resulting in the production of abnormally large red blood cells.

Anemia of Chronic Disease: Chronic diseases, such as rheumatoid arthritis, chronic infections, or malignancies, can lead to macrocytosis due to the body's inflammatory response and subsequent impact on erythropoiesis.
Hypothyroidism: Thyroid hormone plays a crucial role in erythropoiesis. Hypothyroidism can lead to a decrease in erythropoietin production, resulting in macrocytosis.
Alcohol Abuse: Although less common in adolescents, alcohol abuse can cause macrocytosis due to its toxic effects on the bone marrow.

Bone Marrow Failure Syndromes (e.g., Aplastic Anemia, Myelodysplastic Syndromes): These conditions can present with macrocytosis and are potentially life-threatening if not promptly diagnosed and treated.
Leukemia or Lymphoma: Certain types of leukemia or lymphoma can cause macrocytosis, and early detection is crucial for effective treatment.
Inherited Disorders (e.g., Congenital Dyserythropoietic Anemia): Although rare, these disorders can present with macrocytosis and have significant implications for the patient's health and family planning.

Ornithine Transcarbamylase Deficiency: A rare genetic disorder that can lead to macrocytosis due to impaired urea cycle function.
Lesch-Nyhan Syndrome: A genetic disorder characterized by uric acid overproduction, which can cause macrocytosis.
Certain Medications (e.g., Methotrexate, Phenytoin): Certain medications can cause macrocytosis as a side effect, although this is less common in adolescents.