**Question:** A 35-year-old lady with normal prothrombin time (PT) and increased activated partial thromboplastin time (aPTT). Two years ago, she underwent cholecystectomy without any bleeding episode. What is the next investigation for clinical diagnosis?

A. Prothrombin time (PT)
B. Activated partial thromboplastin time (aPTT)
C. Antiphospholipid antibodies
D. Factor V Leiden mutation

**Correct Answer:** C. Antiphospholipid antibodies

**Core Concept:**

The question is discussing a patient with normal PT and increased aPTT, suggesting a factor affecting the intrinsic or common pathway of the coagulation cascade. The intrinsic pathway involves factors 1-10, while the common pathway involves factors 2-9, which are activated by the intrinsic pathway.

**Why the Correct Answer is Right:**

The correct answer, antiphospholipid antibodies, is related to the common pathway, as they can cause aPTT prolongation by binding to phospholipids, which are involved in the activation of factor V and factor X. This results in a decreased rate of the intrinsic pathway and a prolonged aPTT.

**Why Each Wrong Option is Incorrect:**

A. PT: This test measures clotting times for the intrinsic pathway. Since the patient has normal PT, this option is incorrect.
B. aPTT: As explained earlier, the patient has an increased aPTT, not a normal one. This option is incorrect as well.
D. Factor V Leiden mutation: This option concerns a genetic mutation affecting the common pathway, not the common pathway itself or any factor in the intrinsic pathway.

**Clinical Pearl:**

Antiphospholipid antibodies are a group of autoantibodies that can be associated with various autoimmune disorders, including systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). These antibodies can cause aPTT prolongation by binding to phospholipids and affecting the activation of factors V and X, thus disrupting the intrinsic pathway and leading to prolonged aPTT. This is a common finding in patients with antiphospholipid antibodies.

In conclusion, measuring antiphospholipid antibodies is crucial in patients with prolonged aPTT, as it can provide valuable information regarding the presence of antiphospholipid antibodies and guide further management decisions, such as anticoagulation strategies.