Select the Next Best Test to Run Given the Following

Select the next best test to run given the following PTT results on a patient's plasma:
$\begin{array}{ll}\text { Initial PTT: } & 75 \mathrm{~seconds} \\ \text { Immediate } 1: 1 \mathrm{mix} \text { with normal plasma: } & 38 \mathrm{~seconds} \\ \text { Incubated } 1: 1 \mathrm{mix} \text { with normal plasma: } & 36 \mathrm{~seconds}\end{array}$

A) dilute Russell's viper venom time
B) factor assays
C) factor V Leiden
D) Bethesda titer