**Core Concept**
Disseminated Intravascular Coagulation (DIC) is a complex disorder characterized by the systemic activation of coagulation and fibrinolysis, leading to the consumption of platelets and coagulation factors, and the formation of microthrombi in small blood vessels. This results in a paradoxical state of both thrombosis and bleeding.

**Why the Correct Answer is Right**
The most diagnostic feature of DIC is the presence of a significant decrease in fibrinogen levels, often below 150 mg/dL, along with a prolonged prothrombin time (PT) and activated partial thromboplastin time (aPTT). This is due to the consumption of fibrinogen and coagulation factors in the formation of microthrombi. Additionally, the presence of schistocytes (fragmented red blood cells) and microangiopathic hemolytic anemia (MAHA) in DIC is indicative of the microvascular thrombosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Prolonged PT and aPTT are not specific to DIC and can be seen in other conditions such as liver disease or warfarin therapy.
**Option B:** Low platelet count (thrombocytopenia) is a feature of DIC, but it is not the most diagnostic feature.
**Option C:** Elevated D-dimer levels are seen in DIC, but they are also elevated in other conditions such as deep vein thrombosis or pulmonary embolism.

**Clinical Pearl / High-Yield Fact**
A key feature of DIC is the simultaneous presence of thrombosis and bleeding, often in the same patient.

**Correct Answer: D. Decreased fibrinogen levels with prolonged PT and aPTT.**