A 33-year-old man has experienced multiple nosebleeds along with bleeding gums for the past month. On examination, his temperature is 37.3deg C. He has multiple cutaneous ecchymoses. Laboratory studies show hemoglobin, 8.5 g/dL; hematocrit, 25.7%; platelet count, 13,000/mm3; and WBC count, 52,100/mm3 with 5% segmented neutrophils, 5% bands, 2% myelocytes, 83% blasts, 3% lymphocytes, and 2% monocytes. Examination of his peripheral blood smear shows the blasts have delicate nuclear chromatin along with fine cytoplasmic azurophilic granules. These blasts are CD33+. Which of the following morphologic findings is most likely to be present on his peripheral blood smear?

**Core Concept**
The patient's presentation is suggestive of acute myeloid leukemia (AML), a clonal disorder of hematopoietic stem cells characterized by the accumulation of myeloblasts in the bone marrow and peripheral blood. The morphologic findings in AML are diverse and can be associated with various subtypes, including myeloblastic, monocytic, erythroblastic, and megakaryoblastic forms.

**Why the Correct Answer is Right**
The patient's peripheral blood smear shows blasts with delicate nuclear chromatin and fine cytoplasmic azurophilic granules, which is consistent with the diagnosis of AML. The presence of azurophilic granules is a characteristic feature of myeloid blasts, and the CD33 positivity further supports this diagnosis. The blasts in AML can exhibit various morphologic features, including Auer rods, which are needle-like inclusions within the cytoplasm of myeloid blasts. Auer rods are typically associated with the myeloblastic subtype of AML and are a key diagnostic feature.

**Why Each Wrong Option is Incorrect**
* **Option A:** Auer rods are not typically associated with acute lymphoblastic leukemia (ALL), which is characterized by the accumulation of lymphoblasts in the bone marrow and peripheral blood. ALL is often associated with a different set of morphologic features, including a high number of lymphoblasts with fine nuclear chromatin and a lack of azurophilic granules.
* **Option B:** The presence of Auer rods is not a feature of chronic myeloid leukemia (CML), which is a clonal disorder of hematopoietic stem cells characterized by the accumulation of mature granulocytes in the bone marrow and peripheral blood. CML is often associated with a different set of morphologic features, including a high number of mature granulocytes with a left shift.
* **Option C:** Auer rods are not typically associated with acute megakaryoblastic leukemia (AMKL), which is a clonal disorder of hematopoietic stem cells characterized by the accumulation of megakaryoblasts in the bone marrow and peripheral blood. AMKL is often associated with a different set of morphologic features, including a high number of megakaryoblasts with a characteristic "lace-like" nuclear pattern.
* **Option D:** Auer rods are not typically associated with acute erythroid leukemia (AEL), which is a clonal disorder of hematopoietic stem cells characterized by the accumulation of erythroblasts in the bone marrow and peripheral blood. AEL is often associated with a different set of morphologic features, including a high number of erythroblasts with a characteristic "ring sideroblast" appearance.

**Clinical Pearl / High-Yield Fact**
Auer rods are a key diagnostic feature of myeloblastic AML and are often associated with a poor prognosis. They are typically found in the cytoplasm of myeloid blasts and can be stained with special dyes, such as Sudan black B or myeloperoxidase.

**Correct Answer:** C. Auer rods are needle-like inclusions within the cytoplasm of myeloid blasts that are often associated with the myeloblastic subtype of AML.

✓ Correct Answer: A. Auer rods