Ans. is 'b' i.e. an immature T cell phenotype. Clues given in the question are -Increased leucocyte count with blasts constituting 80% of the cellsPresence of mediastinal mass on chest X-ray.Adolescent maleThese findings are suggestive of acute lymphoblastic leukemia of the T cell lineage.Increased leucocyte count in the range of 138 x 109/L and on peripheral blood examination 80% of them constituting blast cells indicate acute leukemia.The age group (adolescent) and the mediastinal mass suggests that this leukemia is in all likelihood a T cell leukemia.Robbin's says "T cell ALL tends to present in adolescent males as lymphomas often with thymic involvement"So it is almost confirmed that the diagnosis is T cell ALL.Now, the immunophenotype required to diagnose T cell ALL.Terminal deoxynucleotidyl transferase (Tdt)It is expressed by both Pre B and Pre T lymphoblasts in > 95% cases, so distinction between ALLs require staining for additional lineage specific markers.CD1, CD2, CDS, CD7It is positive in most cases of precursor T cells.CD3, CD4, CD8Early pre T cells are negative for these markers.Late pre T cells are positive for these markers.Immuno phenotypic classification of acute lymphoblastic leukemiaPre T cell ALLTdT, CD2, CD3, CD5, CD4, CD7, CD8Early pre BTdT, DR, CD 10, CD19, CD24Pre B cellTdT, DR, CD 10, CD19, CD20, CD24, CIgB CellDR, CD 19, CD20, CD24, SIg