Question

A 42 year old man was referred with a 2 week history of fever, weakness, and bleeding gums. Peripheral smear showed pancytopenia. The bone marrow examination revealed 26% blasts, frequently exhibiting Auer rods, and mature myeloid cells. An occasional neutrophil with the Pelger-Huet anomaly was also noted. Which of the following cytochemical stains is most likely to be positive?

• A. Acid phosphatase
• B. Nonspecific esterase
• C. Myeloperoxidase
• D. Toluidine Blue

Hint:

Auer rods are myeloperoxidase-rich; their presence predicts a positive MPO stain.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The clinical picture is a 42 year old man with fever, weakness, and bleeding gums, pancytopenia on smear, and a bone marrow with 26 percent blasts, frequent Auer rods, mature myeloid cells, and a neutrophil with the Pelger-Huet anomaly. We must decide which cytochemical stain will most likely be positive in this marrow.

Step 2: Key Formula or Approach:
Blasts above 20 percent with Auer rods point to a myeloid blast population, which by current WHO criteria falls under acute myeloid leukemia with myelodysplasia related changes rather than the older category of refractory anemia with excess blasts in transformation. Cytochemical stains help confirm the lineage of blasts: myeloperoxidase and Sudan Black B stain myeloid blasts and are strongly positive wherever Auer rods are seen, since Auer rods themselves are made of fused primary granules rich in myeloperoxidase. Nonspecific esterase marks monocytic blasts, acid phosphatase is used mainly for hairy cell leukemia and some T cell lineage markers, and toluidine blue stains mast cell and basophil granules.

Step 3: Detailed Explanation:
Auer rods are needle shaped, crystalline structures made of condensed primary granules, and they are strongly myeloperoxidase positive, this is in fact the classic teaching point, the presence of Auer rods is direct evidence of myeloid differentiation and predicts a positive myeloperoxidase stain.
The mature myeloid cells and the Pelger-Huet neutrophil in the marrow also support a myeloid lineage process, not a lymphoid or monocytic one.
Acid phosphatase would be relevant if we suspected hairy cell leukemia or a T lymphoblastic process, neither fits this myeloid picture.
Nonspecific esterase would be positive in monocytic or myelomonocytic leukemia, but there is no mention of monocytic morphology here, and the Auer rods point away from a purely monocytic process.
Toluidine blue stains basophilic or mast cell granules and is not relevant to a myeloblastic marrow with Auer rods.
Note on the source key: the original paper's explanation only describes the disease as RAEB-T without stating an explicit answer letter. Based on the marrow findings, particularly the Auer rods, the cytochemical stain most likely to be positive is myeloperoxidase, so that is the answer used here.

Step 4: Final Answer:
The cytochemical stain most likely to be positive in this myeloblastic marrow with Auer rods is myeloperoxidase.