**Core Concept**
Bernard-Soulier syndrome is a rare genetic disorder characterized by thrombocytopenia (low platelet count) and giant platelets. It is caused by mutations in the GPIX, GPIa/IIa, or GP1bα subunits of the platelet glycoprotein Ib-IX-V complex, which plays a crucial role in platelet adhesion to collagen and von Willebrand factor (vWF) under high shear stress conditions.

**Why the Correct Answer is Right**
The correct answer, **A. Ristocetin aggregation is normal**, is incorrect because platelets from patients with Bernard-Soulier syndrome exhibit impaired ristocetin-induced platelet aggregation (RIPA). RIPA is a test that assesses the ability of platelets to aggregate in response to the antibiotic ristocetin, which mimics the effect of vWF on platelets. In Bernard-Soulier syndrome, the abnormal glycoprotein Ib-IX-V complex fails to bind to vWF, leading to impaired platelet aggregation.

**Why Each Wrong Option is Incorrect**
**Option B:** Aggregation with collagen and ADP is normal in patients with Bernard-Soulier syndrome, as these pathways are mediated by different receptors and are not affected by the glycoprotein Ib-IX-V complex mutations.

**Option C:** Large platelets are a hallmark of Bernard-Soulier syndrome, resulting from the abnormal glycoprotein Ib-IX-V complex, which disrupts normal platelet production and morphology.

**Option D:** Thrombocytopenia is a characteristic feature of Bernard-Soulier syndrome, resulting from the impaired platelet production and increased platelet destruction.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of Bernard-Soulier syndrome is the presence of giant platelets, which can be identified on a peripheral blood smear. This finding, combined with a history of bleeding symptoms and thrombocytopenia, can aid in the diagnosis of this rare disorder.

**Correct Answer:**
✓ Correct Answer: A. Ristocetin aggregation is normal