Q. A 2-year-old boy is brought to the clinic due to painful knee after he fell on the ground while he was playing with his brother. On examination he has a red, swollen knee. His mother mentions that he has history of prolonged bleeding after he gets injured for any reason. Family history reveals that his maternal uncle has a bleeding problem, as well. Arthrocentesis of the knee revealed bloody fluid. Additional laboratory testing is shown below (the partial thromboplastin time (PTT) is prolonged, other testing is normal). What is the most likely diagnosis?