**Core Concept**
Coagulation failure can be due to various reasons including liver disease, disseminated intravascular coagulation (DIC), and severe trauma. Fibrinogen is a crucial protein in the coagulation cascade, and its levels are often used as a marker of coagulation status. The normal serum level of fibrinogen is between 200-400 mg/dL.

**Why the Correct Answer is Right**
In coagulation failure, the serum level of fibrinogen is often decreased due to consumption of clotting factors. A level less than 100 mg/dL is generally considered critical and indicates severe coagulopathy. This is because fibrinogen is an acute-phase reactant and its levels can drop significantly in cases of severe inflammation, infection, or trauma.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a level of 400 mg/dL is within the normal range for fibrinogen.
**Option B:** This option is also incorrect as 200 mg/dL, although at the lower end of normal, does not indicate coagulation failure.
**Option D:** This option is incorrect because while a level of 50 mg/dL would indicate coagulation failure, the question asks for the threshold below which coagulation failure is indicated.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that fibrinogen levels can be influenced by various factors, including liver disease, nephrotic syndrome, and malnutrition. A low fibrinogen level can indicate coagulopathy, but it's crucial to consider the clinical context and other laboratory findings.

**Correct Answer:** 100 mg/dL