Final answer:
The aPTT test is the fastest way to screen for factor VIII deficiency as it evaluates the intrinsic and common coagulation pathways, which involve factor VIII.
Step-by-step explanation:
The fastest test to determine factor VIII deficiency is b) aPTT (Activated Partial Thromboplastin Time). While FVIII assay is the most specific test for factor VIII levels, aPTT is commonly used as a quick screening test because it evaluates the intrinsic and common pathways of the coagulation cascade. Since factor VIII is part of the intrinsic pathway, a prolonged aPTT can indicate a deficiency in factor VIII. In contrast, PT (Prothrombin Time) and INR (International Normalized Ratio) primarily assess the extrinsic pathway and are not as sensitive to factor VIII levels.