**Core Concept**
Diagnosing Disseminated Intravascular Coagulation (DIC) requires a comprehensive evaluation of coagulation parameters. DIC is characterized by a complex interplay between coagulation and fibrinolysis, leading to widespread microvascular thrombosis and hemorrhage.

**Why the Correct Answer is Right**
Increased fibrin degradation products (FDPs) are a hallmark of DIC. FDPs, such as D-dimer, are generated when fibrin clots are broken down by plasmin, a key enzyme in the fibrinolytic pathway. Elevated FDPs indicate an imbalance between coagulation and fibrinolysis, which is a key feature of DIC. This is because in DIC, the body's natural response to thrombosis is overactivated, leading to excessive fibrin formation followed by breakdown, resulting in increased FDPs.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased PT (prothrombin time) is a common finding in DIC, but it is not specific for the condition. Many other coagulopathies, such as vitamin K deficiency or warfarin therapy, can also prolong PT.

**Option B:** Increased aPTT (activated partial thromboplastin time) can also be seen in DIC, but like PT, it is not specific for the condition. aPTT can be prolonged in a variety of coagulopathies, including heparin therapy or lupus anticoagulant syndrome.

**Option C:** Decreased fibrinogen is a feature of DIC, but it is not the best test for diagnosing the condition. Fibrinogen levels can be decreased in a variety of conditions, including liver disease or severe inflammation.

**Clinical Pearl / High-Yield Fact**
In DIC, the International Society on Thrombosis and Haemostasis (ISTH) scoring system recommends evaluating multiple parameters, including FDPs, PT, aPTT, and fibrinogen levels. A high ISTH score is associated with a higher risk of mortality and complications.

✓ Correct Answer: D. Increased fibrin degradation products