**Core Concept**
Factor VIII is a key cofactor in the intrinsic pathway of coagulation, and its deficiency leads to hemophilia A. The APTT (Activated Partial Thromboplastin Time) measures the time it takes for clot formation in plasma, primarily assessing the function of factors VIII and IX, which are part of the intrinsic pathway.

**Why the Correct Answer is Right**
APTT is sensitive to deficiencies of factors VIII and IX, including factor VIII deficiency (hemophilia A). When factor VIII is absent or low, the intrinsic pathway is impaired, resulting in a prolonged APTT. This test is specifically used to screen for such deficiencies, and correction with normal plasma can confirm the diagnosis. Unlike PT, which assesses the extrinsic and common pathways, APTT directly reflects the intrinsic pathway where factor VIII plays a crucial role.

**Why Each Wrong Option is Incorrect**
Option A: PT measures the extrinsic and common pathways and is not affected by factor VIII deficiency. Thus, it remains normal or only mildly prolonged, making it unsuitable for detecting factor VIII deficiency.
Option C: D-dimer detects fibrin degradation products and is used to assess thrombosis or fibrinolysis, not coagulation factor levels.
Option D: FDP (fibrin degradation products) is a marker of fibrinolysis, not a test for factor VIII activity.

**Clinical Pearl / High-Yield Fact**
APTT is the gold standard for screening hemophilia A; a prolonged APTT with correction by normal plasma confirms factor VIII deficiency. Always perform APTT before testing for factor VIII activity to avoid false negatives.

✓ Correct Answer: B. APTT