Vitamin B12 or Folate Deficiency: This condition often presents with macrocytic anemia (indicated by low Hgb and high MCV), and can also cause leukopenia (low WBC count). The normal platelet count suggests that the bone marrow is not completely failing, which aligns with a nutritional deficiency rather than a more severe marrow disorder.

Anemia of Chronic Disease: This could explain the low hemoglobin, and while it more commonly presents with normocytic anemia, some cases can have a macrocytic component. The low WBC could be due to concurrent infection or inflammation.
Hypersplenism: This condition can cause pancytopenia (low counts of all blood cell types), but the platelet count here is normal. However, it's possible that the spleen is sequestering red and white blood cells, leading to their decreased counts in the peripheral blood.
Medication-Induced Bone Marrow Suppression: Certain medications can suppress the bone marrow, leading to low blood cell counts. The normal platelet count might suggest that the effect is not uniform across all cell lines.

Aplastic Anemia: Although the platelet count is normal, aplastic anemia can present with varying degrees of cytopenias. It's crucial to not miss this diagnosis due to its severe implications and the need for prompt treatment, including potential bone marrow transplantation.
Paroxysmal Nocturnal Hemoglobinuria (PNH): This rare, acquired, life-threatening disease of the blood is characterized by the destruction of red blood cells, bone marrow failure, and the potential for thrombotic events. The normal platelet count does not rule out PNH, and it's a diagnosis that should be considered to avoid missing a potentially fatal condition.
Leukemia or Lymphoma: Although less likely given the specific counts, these malignancies can cause cytopenias due to bone marrow infiltration. Missing a diagnosis of leukemia or lymphoma could have severe consequences.

Pure Red Cell Aplasia: A rare condition where the bone marrow fails to produce red blood cells, but white blood cell and platelet production are unaffected. The low WBC count in this case makes this less likely, but it's a consideration in the differential for the anemia.
Congenital Dyserythropoietic Anemias: These are a group of rare genetic disorders characterized by anemia and other abnormalities in red blood cell production. They might present with macrocytic anemia and could be considered if other causes are ruled out.
Myelodysplastic Syndromes (MDS): A group of disorders caused by poorly formed or dysfunctional blood cells, which could explain the cytopenias. However, MDS more commonly presents with additional features such as ringed sideroblasts in the bone marrow.