In the maxillofacial department a 6-month-old boy was hospitalized due to bleeding from a wound in the frenulum of his tongue. He has a medical history of frenulotomy the day before. The bleeding started 6 hours after the surgery. Suturing the wound was ineffective against the bleeding. The child was brought to the hematology department, where the bleeding was stopped by intravenous administration of coagulation factor IX concentrate. Make the diagnosis: