Soru

A $32$-year-old woman is evaluated in the emergency department for right lower extremity swelling and severe calf pain that started $2$ days prior. Her medical history is significant for $3$ consecutive first-trimester pregnancy losses, but she is otherwise healthy. She takes no medications and has no recent history of surgery, trauma, or prolonged travel. Her vital signs are temperature $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$), blood pressure $118/76\text{ mmHg}$, pulse $88\text{/min}$, and respirations $14\text{/min}$. Physical examination reveals a swollen, erythematous, and tender right calf. A Doppler ultrasound of the right leg confirms a popliteal deep venous thrombosis. Initial laboratory tests show a normal prothrombin time (PT) but an activated partial thromboplastin time (aPTT) of $62\text{ seconds}$ (reference range: $25-35\text{ seconds}$). Which of the following is the most appropriate next step in the laboratory evaluation of this patient's abnormal coagulation profile?