**Core Concept**
Primary coagulation defects, also known as bleeding disorders, are conditions characterized by an abnormality in the blood's ability to form clots. These defects can be caused by deficiencies or dysfunction in various coagulation factors. Preliminary investigations in primary coagulation defects involve tests that assess the overall coagulation status of the patient.

**Why the Correct Answer is Right**
The correct preliminary investigations in primary coagulation defects include:
- **Activated Partial Thromboplastin Time (aPTT):** measures the time it takes for plasma to clot after the addition of a substance that activates the intrinsic coagulation pathway.
- **Prothrombin Time (PT):** measures the time it takes for plasma to clot after the addition of tissue factor, which activates the extrinsic coagulation pathway.
- **Platelet count:** assesses the number of platelets in the blood, which play a crucial role in primary hemostasis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not typically a preliminary investigation in primary coagulation defects.

**Option B:** This option is not a preliminary investigation in primary coagulation defects.

**Option C:** This option is not a preliminary investigation in primary coagulation defects.

**Clinical Pearl / High-Yield Fact**
When evaluating primary coagulation defects, it's essential to remember that a prolonged aPTT is often associated with deficiencies in the intrinsic coagulation pathway, while a prolonged PT is associated with deficiencies in the extrinsic coagulation pathway.

**Correct Answer:** A