**Core Concept**
Thrombocytopenia, characterized by a platelet count below 150,000/μL, is a critical condition that can lead to bleeding manifestations such as petechiae, purpura, and epistaxis. In children, the most common causes of thrombocytopenia include infections, autoimmune disorders, and hematologic malignancies.

**Why the Correct Answer is Right**
The patient's laboratory results show a significant decrease in platelet count (2000/mm3), which is consistent with thrombocytopenia. In a healthy 3-year-old girl, the most likely cause of acute onset thrombocytopenia is a viral infection, such as chickenpox or measles. These infections can lead to immune-mediated thrombocytopenia, where the body's immune system mistakenly attacks and destroys platelets. The presence of petechiae, purpura, and epistaxis further supports this diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thrombotic thrombocytopenic purpura (TTP) is a rare disorder characterized by thrombocytopenia, microangiopathic hemolytic anemia, and renal failure. While TTP can present with thrombocytopenia, the acute onset of petechiae, purpura, and epistaxis in a healthy child is not typical for this condition.
* **Option B:** Leukemia is a malignancy of the hematopoietic system that can present with thrombocytopenia. However, leukemia typically presents with a more gradual onset of symptoms, including fatigue, weight loss, and lymphadenopathy, which are not mentioned in this case.
* **Option C:** Disseminated intravascular coagulation (DIC) is a condition characterized by both thrombosis and bleeding due to the activation of the coagulation cascade. While DIC can present with thrombocytopenia, the acute onset of petechiae, purpura, and epistaxis in a healthy child is not typical for this condition.

**Clinical Pearl / High-Yield Fact**
In children, immune-mediated thrombocytopenia is often associated with viral infections, and the diagnosis can be supported by the presence of petechiae, purpura, and epistaxis. A thorough history and physical examination, along with laboratory tests, are essential for making a diagnosis.

**Correct Answer:** D.