FRCPath Haem Part 1 MCQ-Gen Haem 201
A patient’s bone marrow shows blasts that are negative for myeloperoxidase and Sudan black B but strongly positive for PAS. Which lineage is most likely?
Myeloid
Monocytic
Megakaryocytic
Lymphoid
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1 Comment
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Correct Answer: Lymphoid
This pattern is characteristic of lymphoid lineage, specifically acute lymphoblastic leukaemia (ALL) — most often B-lymphoblastic leukaemia.
Explanation:
Myeloperoxidase (MPO) and Sudan Black B (SBB) → mark myeloid differentiation → negative in ALL.
Periodic acid–Schiff (PAS) → shows block cytoplasmic positivity in lymphoblasts (especially B-ALL).
Erythroblasts in pure erythroid leukaemia can also be PAS+, but they usually show a diffuse, granular pattern, not block-type.