**Question:** A 45-year-old lady presents with normal prothrombin time (PT) and increased activated partial thromboplastin time (aPTT). About 2 years ago, she was operated for cholecystectomy and did not have any bleeding episode. What is the next investigation for clinical diagnosis?

A. Thrombocytopenia
B. Factor V Leiden mutation
C. Hemostatic panel (PT, aPTT, fibrinogen, D-dimer)
D. Vitamin K deficiency

**Correct Answer:** C. Hemostatic panel (PT, aPTT, fibrinogen, D-dimer)

**Core Concept:** Increased aPTT with normal PT suggests a hemostatic disorder or drug interference. Hemostatic panel is a comprehensive test to evaluate the coagulation cascade and clotting factors.

**Why the Correct Answer is Right:** Hemostatic panel, including PT, aPTT, fibrinogen, and D-dimer, is essential to further evaluate the patient's hemostatic profile. These tests assess different aspects of the clotting cascade:

1. Prothrombin Time (PT): Evaluates the extrinsic pathway and vitamin K-dependent factors (II, VII, IX, X, and clotting factor proteins C and S).
2. Activated partial thromboplastin time (aPTT): Assesses the intrinsic pathway, common pathway, and factor VIII/IX clotting factors.
3. Fibrinogen: Indicates the level of fibrinogen, a key protein in blood clot formation.
4. D-dimer: Measures the degradation product of cross-linked fibrin, which can be elevated in conditions that cause increased fibrinolysis (e.g., thrombosis, disseminated intravascular coagulation, or acute inflammation).

**Why Each Wrong Option is Incorrect:**

A. Thrombocytopenia is not the primary issue in this case, as the PT is normal, ruling out severe platelet dysfunction.
B. Factor V Leiden mutation (an inherited thrombophilia) is not directly related to increased aPTT and normal PT, as it primarily affects the coagulation cascade and is associated with prolonged aPTT.

C. Vitamin K deficiency (as in warfarin therapy) could lead to increased aPTT and decreased PT, not the other way around.

D. Vitamin K deficiency is not the only cause of increased aPTT and decreased fibrinogen, as it only affects the extrinsic pathway and vitamin K-dependent factors.

**Clinical Pearl:** A thorough evaluation of hemostatic profile helps identify potential causes of abnormal coagulation parameters. In this case, ordering a hemostatic panel will guide towards the underlying cause of increased aPTT, such as a bleeding disorder or drug interference, and assess fibrinogen levels. This will help in decision-making regarding further investigations, management, and prognosis for the patient.