**Core Concept**
Immune thrombocytopenic purpura (ITP) is a bleeding disorder characterized by isolated thrombocytopenia, resulting from immune-mediated destruction of platelets. The underlying mechanism involves autoantibody production against platelet antigens, leading to platelet opsonization and subsequent phagocytosis by splenic macrophages.

**Why the Correct Answer is Right**
In ITP, the autoantibodies bind to platelet surface antigens, such as GPIIb/IIIa, marking them for destruction. This process is mediated by the Fc receptor on splenic macrophages, which recognize the IgG antibodies and engulf the opsonized platelets. The resulting thrombocytopenia leads to an increased risk of bleeding, including petechiae, purpura, and mucocutaneous bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** While ITP can present with anemia and leukopenia, these findings are not characteristic of the disease.

**Option B:** The presence of a platelet count below 100,000/μL is a diagnostic criterion for ITP, but it is not a defining feature of the disease.

**Option C:** Splenomegaly is often seen in ITP, but it is not a universal finding and can be absent in some cases.

**Clinical Pearl / High-Yield Fact**
The diagnosis of ITP is primarily based on the exclusion of other causes of thrombocytopenia, such as bone marrow failure, and the demonstration of isolated thrombocytopenia with a normal bone marrow morphology.

**Correct Answer: B.** The presence of a platelet count below 100,000/μL is a diagnostic criterion for ITP.