A 5-year-old boy visits his primary care physician after an accident on his bike. It was his first accident, and he fell on his knees. His mother noticed severe swelling in his knees and elbows but thought that it was from the accident

She placed an ice pack on the affected areas and put him to rest. After a few days, the swelling had not improved, so she made an appointment to see his physician who asks him a few questions about the accident and then proceeds to perform a physical examination. The physician notes small hematomas on his elbows, knees, and his upper torso. The results from the blood draw follow.

PT: 12.2 sec (RR: 11.3-12.8 sec)
INR: 1.12
APTT: 72 sec (RR: 23-32 sec)
Platelet aggregation studies: normal aggregation with all agonists

Predict results that will confirm the patient's diagnosis.
a. Abnormal aggregation with Ristocetin
b. An increased D-dimer
c. Decreased activity of either factors VIII or IX
d. The presence of an inhibitor pattern in a mixing study

Correct Answer: C