**Core Concept**
Acute Myeloid Leukemia (AML) is a heterogeneous group of clonal hematopoietic malignancies characterized by the uncontrolled proliferation of myeloid blasts in the bone marrow. The prognosis of AML varies widely depending on several factors, including the specific subtype, age, and molecular characteristics of the disease.

**Why the Correct Answer is Right**
The worst prognosis in AML is associated with the **M4Eo subtype**, also known as acute myelomonocytic leukemia with eosinophilia. This subtype is characterized by the presence of eosinophilia and monocytic differentiation, which is often accompanied by a complex karyotype and the presence of adverse genetic mutations, such as **FLT3-ITD**. The M4Eo subtype is more common in adults and has a poor response to chemotherapy, resulting in a lower overall survival rate.

**Why Each Wrong Option is Incorrect**
* **Option A:** M3 (Acute Promyelocytic Leukemia) is actually considered to have a relatively good prognosis, especially with the use of all-trans retinoic acid (ATRA) and arsenic trioxide.
* **Option B:** M2 (Acute Myeloid Leukemia with Maturation) is a common subtype of AML, but it is not associated with the worst prognosis.
* **Option C:** M5 (Acute Monocytic Leukemia) is another subtype of AML, but it is not as aggressive as M4Eo.

**Clinical Pearl / High-Yield Fact**
The presence of **FLT3-ITD** mutations is a significant adverse prognostic factor in AML, and it is often associated with a higher risk of relapse and a shorter overall survival.

**Correct Answer:** D.