Which morphologic feature best distinguishes acute promyelocytic leukemia (APL/AML-M3) from acute myeloblastic leukemia (AML-M1)?

Question

Accepted Answer

A. Abnormal promyelocytes with heavy azurophilic granulation and multiple Auer rods (often in bundles). ## Morphologic Distinction: APL vs AML-M1

### Key Morphologic Features

| Feature | APL (AML-M3) | AML-M1 (Myeloblastic) |
|---------|--------------|----------------------|
| **Cell type** | Abnormal promyelocytes | Myeloblasts |
| **Granulation** | Heavy azurophilic (abundant) | Minimal to absent |
| **Auer rods** | Multiple, often in bundles ("faggot cells") | Single or absent |
| **Cytoplasm** | Abundant, heavily granulated | Scanty to moderate |
| **Nucleoli** | Prominent | Prominent |
| **Cytochemistry (MPO)** | Strongly positive | Positive |

**Key Point:** APL is characterized by abnormal promyelocytes with heavy azurophilic granulation and distinctive multiple Auer rods often arranged in bundles (pathognomonic finding called "faggot cells"). This is the single most discriminating morphologic feature.

**Clinical Pearl:** APL (AML-M3) is unique among acute leukemias because it is highly associated with disseminated intravascular coagulation (DIC) due to release of procoagulant substances from abnormal promyelocyte granules. This clinical presentation, combined with the distinctive morphology, makes it immediately recognizable.

**High-Yield:** The bundle of Auer rods in APL is so characteristic that its presence should immediately raise suspicion for APL and prompt urgent coagulation studies and consideration of ATRA (all-trans retinoic acid) therapy, which has dramatically improved outcomes in this previously fatal leukemia.

**Mnemonic:** **APL = Abundant Promyelocytes with Lots of granules and multiple Auer rods**

Answer

B. Predominance of myeloblasts with minimal granulation

Answer

C. Presence of monocytic differentiation with abundant cytoplasm

Answer

D. Presence of abundant Auer rods in blast cytoplasm

Which morphologic feature best distinguishes acute promyelocytic leukemia (APL/AML-M3) from acute myeloblastic leukemia (AML-M1)?

Explanation

Morphologic Distinction: APL vs AML-M1

Key Morphologic Features

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Feature	APL (AML-M3)	AML-M1 (Myeloblastic)
Cell type	Abnormal promyelocytes	Myeloblasts
Granulation	Heavy azurophilic (abundant)	Minimal to absent
Auer rods	Multiple, often in bundles ("faggot cells")	Single or absent
Cytoplasm	Abundant, heavily granulated	Scanty to moderate
Nucleoli	Prominent	Prominent
Cytochemistry (MPO)	Strongly positive	Positive