You are answering a USMLE-style medical question. Question: A 23-year-old man presents to his primary care physician after bleeding from his gums while brushing his teeth for the past week. Although he has had gum bleeding in the past, it has never been this severe. He has no family history available as he was adopted. He does not have any significant past medical history. His physical exam is within normal limits. Screening lab work is ordered with the following values: Platelet count 330,000/mm3 Bleeding time 6 mins Prothrombin time 12 sec Activated partial thromboplastin time 42 sec Factor VIII activity decreased Ristocetin cofactor assay decreased He is subsequently referred to a hematologist for further work-up and treatment. Which of the following aspects of coagulation would most likely be affected? Options: A) Gp Ib B) Factor XI C) Vitamin K D) Antithrombin III Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.