**Core Concept**
The patient presents with an isolated prolongation of activated partial thromboplastin time (aPTT), which suggests a problem with the intrinsic coagulation pathway. This pathway involves various **coagulation factors**, including factors VIII, IX, XI, and XII.

**Why the Correct Answer is Right**
To evaluate the patient, it's essential to identify the cause of the prolonged aPTT. Given the lack of bleeding tendency and normal prothrombin time (PT) and platelet count, the next step would involve mixing studies to differentiate between a coagulation factor deficiency and an inhibitor. However, since the options are not provided, we can infer that the correct approach would involve further testing to determine the underlying cause of the prolonged aPTT, such as **lupus anticoagulant** or a specific factor deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option text, we cannot directly address why it is incorrect.
**Option B:** Similarly, without the text, we cannot explain its incorrectness.
**Option C:** This option's incorrectness cannot be determined without its content.
**Option D:** The same applies, as the option details are not provided.

**Clinical Pearl / High-Yield Fact**
In patients with isolated prolongation of aPTT, it's crucial to consider **lupus anticoagulant** as a potential cause, especially if there's no significant bleeding history. This condition can be associated with **antiphospholipid syndrome**.

**Correct Answer:** D. Mixing study to distinguish between factor deficiency and inhibitor