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

**Core Concept**
The patient's presentation of fever, weakness, bleeding gums, pancytopenia, and bone marrow findings suggestive of acute myeloid leukemia (AML) with maturation is indicative of a specific subtype of AML. The presence of Auer rods and pseudo-Pelger-Huet anomaly in neutrophils is characteristic of this condition.

**Why the Correct Answer is Right**
The correct answer is related to the cytochemical staining of myeloperoxidase (MPO), which is typically positive in AML with maturation. MPO is an enzyme found in the primary granules of myeloid cells and is essential for the production of hydrogen peroxide, a key component in the elimination of ingested microbes. The presence of MPO-positive blasts in the bone marrow is a hallmark of AML with maturation.

**Why Each Wrong Option is Incorrect**
**Option A:** Sudan Black B staining is used to identify lipid-laden cells, such as those seen in hairy cell leukemia, not AML with maturation.
**Option B:** Periodic Acid-Schiff (PAS) staining is used to identify glycogen-containing cells, such as those seen in acute lymphoblastic leukemia (ALL), not AML with maturation.
**Option C:** Alkaline Phosphatase (AP) staining is used to identify cells with high levels of AP activity, such as those seen in certain types of lymphoma, not AML with maturation.

**Clinical Pearl / High-Yield Fact**
Auer rods are characteristic of acute myeloid leukemia (AML) and are a key feature in the diagnosis of this condition. They are needle-like inclusions found in the cytoplasm of myeloid blasts and are a result of the abnormal accumulation of azurophilic granules.

**Correct Answer:** C. Myeloperoxidase (MPO)

✓ Correct Answer: C. Myeloperoxidase