**Core Concept**
Disseminated intravascular coagulation (DIC) is a complex condition characterized by both widespread clotting and bleeding in the vascular system, resulting from an imbalance in the coagulation and anticoagulation pathways. The diagnosis of DIC involves various laboratory tests to assess coagulation parameters.

**Why the Correct Answer is Right**
The most specific investigation for diagnosing DIC involves assessing the levels of **D-dimer**, a fibrin degradation product that indicates recent clotting activity. Elevated D-dimer levels are highly suggestive of DIC, as they reflect the consumption of clotting factors and the formation of fibrin, which is then broken down into D-dimers.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while platelet count is important in DIC, thrombocytopenia can be seen in various conditions, not just DIC.
**Option B:** Incorrect as PT/INR can be abnormal in many coagulopathies, not specifically indicating DIC.
**Option C:** Incorrect because aPTT can be prolonged in several conditions affecting the intrinsic and common coagulation pathways, not exclusively in DIC.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that DIC is a clinical diagnosis supported by laboratory findings, including a high D-dimer level, low platelet count, prolonged PT/INR, and decreased fibrinogen levels.

**Correct Answer:** D. D-dimer