Regarding acute myeloid leukemia (AML), all of the following are characteristic features EXCEPT:

Explanation

## Acute Myeloid Leukemia: Diagnostic Features **Key Point:** Auer rods are highly specific for AML when present, but they are NOT pathognomonic and are absent in the majority of AML cases (~70% of cases lack Auer rods). Their absence does not exclude AML. ### Diagnostic Criteria for AML | Feature | Details | |---------|----------| | **Blast percentage** | ≥20% blasts in bone marrow or peripheral blood (WHO 2016 criteria) | | **Auer rods** | Highly specific when present (abnormal azurophilic granules) but NOT required for diagnosis | | **Cytochemistry** | MPO+, Sudan black+, PAS variable | | **Cytogenetics** | t(15;17), t(8;21), inv(16) are recurrent translocations | ### Auer Rods: Why NOT Pathognomonic 1. Present in only ~30% of AML cases 2. When present, highly suggestive of AML (not seen in ALL) 3. Absence does NOT exclude AML diagnosis 4. Found in AML-M3 (APL) most commonly **High-Yield:** The diagnostic threshold of ≥20% blasts is the gold standard for AML diagnosis, regardless of Auer rod presence. **Clinical Pearl:** In acute promyelocytic leukemia (APL), Auer rods are often abundant ("bundles of sticks"), but even in APL, diagnosis relies on the 20% blast criterion and t(15;17) cytogenetics, not Auer rods alone. ### Why Other Options Are Correct - **≥20% blasts:** WHO diagnostic criterion for AML [cite:WHO Classification 2016] - **t(15;17) in APL:** Pathognomonic translocation for AML-M3 [cite:Robbins 10e Ch 13] - **MPO positivity:** Hallmark of myeloid differentiation; AML is MPO+ by definition [cite:Robbins 10e Ch 13]