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 examination revelaed 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 underlying principle being tested is the identification of a specific type of leukemia based on cytochemical staining patterns. In this case, the presence of Auer rods and pseudo Pelger-Huet anomaly in a patient with pancytopenia and bone marrow blasts suggests a diagnosis of acute myeloid leukemia (AML).

**Why the Correct Answer is Right**
The presence of Auer rods in blasts is a characteristic feature of AML, specifically of the M3 subtype (acute promyelocytic leukemia, APL). Auer rods are abnormal azurophilic granules found in the cytoplasm of myeloid cells. The pseudo Pelger-Huet anomaly is a characteristic nuclear feature of AML, where the nucleus of the neutrophil appears bilobed or kidney-shaped. Myeloperoxidase (MPO) is an enzyme found in the azurophilic granules of myeloid cells, and its presence can be demonstrated using the MPO cytochemical stain.

**Why Each Wrong Option is Incorrect**
* **Option A:** Sudan Black B (SBB) stain is used to detect lipids in cells, and it is not specific for AML.
* **Option B:** Alkaline Phosphatase (AP) stain is used to detect the enzyme alkaline phosphatase in cells, but it is not the best choice for AML.
* **Option D:** Periodic Acid-Schiff (PAS) stain is used to detect glycogen in cells, but it is not specific for AML.

**Clinical Pearl / High-Yield Fact**
Auer rods are a key diagnostic feature of AML, and their presence can help differentiate it from other types of leukemia. The MPO cytochemical stain is a useful tool for confirming the diagnosis of AML, especially in cases where Auer rods are present.

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

✓ Correct Answer: C. Myeloperoxidase