**Core Concept**
Postpartum hemorrhage (PPH) is a life-threatening condition characterized by excessive bleeding after childbirth. The patient's laboratory results indicate a prolonged activated partial thromboplastin time (aPTT) and a normal prothrombin time (PT), suggesting a possible coagulopathy. Coagulopathy in PPH can be caused by either coagulation factor deficiency or the presence of a factor inhibitor.

**Why the Correct Answer is Right**
The correct answer involves using a specific test to differentiate between coagulation factor deficiency and factor inhibitor. Factor inhibitors are antibodies that bind to clotting factors, preventing their normal function. In contrast, coagulation factor deficiencies result from a lack of these factors. The test that can help differentiate between these two causes is the mixing study. When a patient's plasma is mixed with normal plasma and the aPTT is reassessed, a correction in aPTT indicates the presence of a factor inhibitor. However, if the aPTT remains prolonged, it suggests a coagulation factor deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is not relevant to the differentiation between coagulation factor deficiency and factor inhibitor.

**Option B:**
This option is not a valid test for differentiating between coagulation factor deficiency and factor inhibitor.

**Option D:**
This option is not a specific test for differentiating between coagulation factor deficiency and factor inhibitor.

**Clinical Pearl / High-Yield Fact**
When interpreting coagulation tests in patients with PPH, it is essential to consider the clinical context and perform further tests, such as the mixing study, to differentiate between coagulation factor deficiency and factor inhibitor.

**Correct Answer:** C. Mixing study.