Which AML subtype is classically associated with Auer rods on smear and high myeloperoxidase activity?

• A. M0
• B. M2
• C. M4
• D. M3

Answer: (D)

Auer rods are needle-like azurophilic granule inclusions that reflect myeloid lineage with abundant primary granules. When you see Auer rods on a smear along with strong myeloperoxidase activity, it points most strongly to acute promyelocytic leukemia, the AML subtype known as M3. This form features promyelocytes packed with primary granules, many of which contain Auer rods, giving both the characteristic rods and high MPO staining. The underlying biology involves the PML-RARA fusion from t(15;17), which blocks maturation and leads to accumulation of these granule-rich promyelocytes. While other AML subtypes can show MPO positivity and occasionally Auer rods, the combination described is classically associated with acute promyelocytic leukemia.