Which condition would most likely present with a prolonged bleeding time and decreased platelet aggregation?

The condition that would most likely present with a prolonged bleeding time and decreased platelet aggregation is von Willebrand's disease. This hereditary bleeding disorder is caused by a deficiency or dysfunction of von Willebrand factor (vWF), which is crucial for platelet adhesion to the site of vascular injury.

In von Willebrand's disease, the absence or dysfunction of vWF leads to inadequate platelet connectivity and aggregation, which contributes to prolonged bleeding times. The condition results in the inability of platelets to effectively adhere to the exposed collagen at injury sites, ultimately affecting hemostasis.

Other conditions listed may also affect bleeding but do not typically present with both prolonged bleeding time and decreased platelet aggregation in the same way. Aspirin ingestion, for instance, inhibits cyclooxygenase and affects platelet function, leading to impaired aggregation but does not primarily cause a deficiency in vWF. Protein C deficiency and Factor X deficiency predominantly impact coagulation cascades, resulting in thrombosis or clotting issues rather than directly influencing platelet function and bleeding time like von Willebrand's disease does.