**Core Concept**
Spontaneous bleeding is a serious complication of thrombocytopenia, which is a condition characterized by low **platelet count**. The risk of spontaneous bleeding increases as the platelet count decreases. This is because **platelets** play a crucial role in the formation of blood clots.

**Why the Correct Answer is Right**
The correct answer is related to the critical level of thrombocytopenia at which spontaneous bleeding typically occurs. When the platelet count falls below a certain threshold, the body's ability to form clots is severely impaired, leading to an increased risk of spontaneous bleeding. This threshold is generally considered to be around **10,000 to 20,000 platelets per microliter** of blood.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a platelet count above 50,000 is generally not associated with a high risk of spontaneous bleeding.
**Option B:** This option is also incorrect because a platelet count above 20,000 but below 50,000 may increase the risk of bleeding, but spontaneous bleeding is less common.
**Option D:** This option is incorrect because a normal platelet count is between 150,000 and 450,000 per microliter, and spontaneous bleeding is rare at this level.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the risk of bleeding also depends on other factors, such as the presence of other bleeding disorders or the use of anticoagulant medications. A key point to recall is that spontaneous bleeding is more likely to occur when the platelet count is very low.

**Correct Answer:** C. 10,000 to 20,000 platelets per microliter