A 42 -year old man was referred with a 2 week history of fever weakness and bleeding gum. Peripherial smear showed pancytopenia. The bone marrow examinationrevelaed 26% blasts frequently exhibiting Auer rods andmature myeloid cells. An occasional neutrophil withpseudo Pelger-Huet anomaly was also noted : Which ofthe following cytocheimcal stains is most likely to bepositive ?

**Core Concept**
The patient's presentation of fever, weakness, bleeding gums, and pancytopenia, along with the bone marrow findings of 26% blasts and Auer rods, suggests a diagnosis of Acute Myeloid Leukemia (AML). Auer rods are characteristic of myeloid lineage cells and are often seen in AML.

**Why the Correct Answer is Right**
The presence of Auer rods in the blasts is a key diagnostic feature of AML. Auer rods are azurophilic granules that are formed within the cytoplasm of myeloid cells. They are composed of aggregated azurophilic granules and are a hallmark of myeloid differentiation. The specific type of Auer rod can help differentiate between subtypes of AML. In this case, the presence of Auer rods suggests a diagnosis of AML with myeloid differentiation.

**Why Each Wrong Option is Incorrect**
* **Option A:** does not have any relevance to the diagnosis of AML.
* **Option B:** is a different type of staining that is not relevant to the diagnosis of AML in this context.
* **Option D:** is not a specific cytochemical stain and is not relevant to the diagnosis of AML.

**Clinical Pearl / High-Yield Fact**
Auer rods are a characteristic feature of AML and can be seen in up to 90% of cases. They are composed of aggregated azurophilic granules and are a key diagnostic feature of myeloid lineage cells.

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

✓ Correct Answer: C. Myeloperoxidase