**Core Concept**
Glanzman thrombasthenia is a rare bleeding disorder characterized by impaired platelet aggregation, leading to prolonged bleeding. It is caused by a deficiency or dysfunction of specific platelet membrane glycoproteins.

**Why the Correct Answer is Right**
Glanzman thrombasthenia is caused by mutations in the genes encoding the alpha IIb subunit (ITGA2B) and the beta 3 subunit (ITGB3) of the glycoprotein IIb/IIIa complex. This complex is essential for platelet aggregation, as it serves as the receptor for fibrinogen, allowing platelets to form clumps. In Glanzman thrombasthenia, the defective glycoprotein IIb/IIIa complex impairs platelet aggregation, leading to bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** (not provided)
**Option B:** This option is incorrect because Glanzman thrombasthenia is not caused by a defect in von Willebrand factor, which is involved in another type of bleeding disorder.
**Option C:** This option is incorrect because Glanzman thrombasthenia is not caused by a defect in the coagulation factors of the intrinsic pathway, which is involved in a different type of bleeding disorder.
**Option D:** (not provided)

**Clinical Pearl / High-Yield Fact**
Glanzman thrombasthenia is characterized by a normal platelet count, but impaired platelet function, leading to prolonged bleeding. It is often diagnosed through a platelet aggregation test.

**Correct Answer: C. ITGB3**