**Core Concept:** Acute Myeloid Leukemia (AML) is a type of blood cancer characterized by the rapid proliferation of abnormal white blood cells, known as blasts, in the bone marrow and blood. The percentage of blasts in the peripheral blood is a crucial diagnostic parameter.

**Why the Correct Answer is Right:** In AML, the blast cells are immature and have not matured into normal myeloid cells. The presence of blasts in the bone marrow and blood samples indicates an abnormal increase in these cells. The correct answer is based on the diagnostic criteria for AML, which include the presence of ≥20% blasts in the peripheral blood.

**Why Each Wrong Option is Incorrect:**

A. **5-20% blasts:** This option is incorrect as it falls short of the diagnostic threshold for AML, which requires ≥20% blasts in the peripheral blood.

B. **Over 20% blasts:** This option is incorrect as it is higher than the diagnostic threshold for AML. AML is defined by the presence of ≥20% blasts in the peripheral blood.

C. **Absence of blasts:** This option is incorrect as AML is characterized by the presence of blasts in the bone marrow and peripheral blood. The absence of blasts indicates a different condition or stage of disease progression.

D. **Undifferentiated blasts:** This option is incorrect as the presence of blasts in AML is characterized by their undifferentiated appearance and failure to mature into normal myeloid cells. The correct answer focuses on the percentage of blasts rather than their differentiation status.

**Clinical Pearl:** A thorough understanding of the diagnostic criteria for AML is essential for accurate diagnosis and appropriate treatment planning. In AML, the presence of ≥20% blasts in the peripheral blood is a key diagnostic criterion. Monitoring the percentage of blasts over time can also be useful in assessing treatment response and disease progression.