**Core Concept**
Acute Myeloid Leukemia (AML) is a heterogeneous group of diseases characterized by the clonal expansion of myeloid blasts in the bone marrow. The prognosis of AML varies significantly depending on several factors, including age, cytogenetic abnormalities, and molecular mutations.

**Why the Correct Answer is Right**
The worst prognosis in AML is associated with the presence of **inv(16)(p13.1q22)**, which involves the CBFB and MYH11 genes. This abnormality is characterized by the formation of the core-binding factor beta (CBFB)-myeloid-specific enhancer factor 2A (MEF2A) fusion protein, leading to the inhibition of normal myeloid differentiation and the promotion of leukemic cell proliferation. Patients with inv(16) have a poor response to chemotherapy and a high risk of relapse.

**Why Each Wrong Option is Incorrect**
**Option A:** t(8;21)(q22;q22) is associated with a favorable prognosis in AML, characterized by the RUNX1-RUNX1T1 fusion protein, which promotes the differentiation of myeloid cells.
**Option B:** NPM1 mutations are common in AML and are associated with a good prognosis, particularly when combined with a normal karyotype.
**Option C:** MLL rearrangements are associated with a poor prognosis in AML, but are more common in infants and younger adults.

**Clinical Pearl / High-Yield Fact**
The presence of a cytogenetic abnormality, such as inv(16), should prompt a discussion about the potential need for allogeneic hematopoietic stem cell transplantation in order to improve outcomes.

**Correct Answer: C. MLL rearrangements are associated with a poor prognosis in AML, but are more common in infants and younger adults.**