## Core Concept
Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease characterized by low platelet count due to immune-mediated destruction. The risk of bleeding in ITP patients is related to the platelet count. Generally, a certain threshold of platelet count is crucial in determining the risk of spontaneous bleeding.

## Why the Correct Answer is Right
The correct answer, **5000 platelets/mm^3**, or 5 x 10^9/L, is significant because it is often cited as the critical threshold below which the risk of spontaneous bleeding, including mucocutaneous and potentially life-threatening hemorrhages, increases substantially. At platelet counts above this threshold, the risk of serious spontaneous bleeding is relatively low, although it does not entirely eliminate the risk. This value helps guide clinical management, including the need for interventions like platelet transfusions in acute settings.

## Why Each Wrong Option is Incorrect
- **Option A: 20,000 platelets/mm^3**: This is too high. While patients with counts below 20,000 platelets/mm^3 are at increased risk of bleeding and may require more careful monitoring or intervention, serious generalized bleeding is less common at this level.
- **Option B: 50,000 platelets/mm^3**: This is also too high. Patients with platelet counts around 50,000/mm^3 may have mild symptoms or be asymptomatic and are often managed conservatively without immediate intervention for bleeding prevention.
- **Option D: 100,000 platelets/mm^3**: This is much too high. A platelet count of 100,000/mm^3 is generally not considered to pose a significant risk for spontaneous bleeding and is within a range where most patients with ITP would not require specific therapy to prevent bleeding.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while the threshold of **5,000 platelets/mm^3** is critical for the risk of generalized bleeding, the management of ITP also depends on the patient's symptoms, the rate of platelet count decline, and the presence of active bleeding. Patients with very low platelet counts may require hospitalization and close monitoring, while those with higher counts but significant bleeding symptoms may also need intervention.

## Correct Answer: C. 5000 platelets/mm^3.