Myelodysplastic Syndrome (MDS): This is the most likely diagnosis given the presence of bicytopenia (involving red blood cells and platelets) and the bone marrow biopsy showing absent megakaryocytes. MDS is a group of disorders caused by poorly formed or dysfunctional blood cells, often leading to bone marrow failure. The absence of megakaryocytes, which are responsible for platelet production, aligns with the thrombocytopenia (low platelet count) observed. The presence of 10% atypical cells on the peripheral smear further supports this diagnosis, as MDS is characterized by dysplastic changes in the blood cells.

Aplastic Anemia: Although less likely than MDS due to the specific absence of megakaryocytes and the presence of atypical cells, aplastic anemia is a condition where the bone marrow fails to produce blood cells. It could present with bicytopenia or pancytopenia but typically shows a more uniform decrease in all cell lines without the dysplastic features seen in MDS.
Myeloproliferative Neoplasm (MPN): Certain MPNs, like essential thrombocythemia or primary myelofibrosis, can present with abnormalities in blood cell counts and bone marrow findings. However, the absence of megakaryocytes and the specific pattern of bicytopenia make this less likely.

Acute Leukemia: Although the bone marrow biopsy and peripheral smear do not strongly suggest acute leukemia, it is crucial not to miss this diagnosis due to its aggressive nature and need for immediate treatment. The presence of 10% atypical cells could be indicative of a leukemic process, especially if these cells have blast characteristics.
Paroxysmal Nocturnal Hemoglobinuria (PNH): PNH is 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. It can present with bicytopenia and would be a critical diagnosis not to miss due to its specific treatment options and potential complications.

Amegakaryocytic Thrombocytopenia: A rare congenital or acquired disorder characterized by the absence of megakaryocytes in the bone marrow, leading to severe thrombocytopenia. While it aligns with the absence of megakaryocytes, the presence of anemia (as part of bicytopenia) and atypical cells makes this a less likely diagnosis.
Large Granular Lymphocyte (LGL) Leukemia: A rare disorder involving the proliferation of LGL cells, which can lead to cytopenias, including anemia and thrombocytopenia, due to bone marrow suppression or immune-mediated destruction of blood cells. The presence of atypical cells could potentially represent LGL leukemia, but specific immunophenotyping would be required for diagnosis.