## Core Concept
The question tests the understanding of coagulation and thrombosis, focusing on the role of various factors in the coagulation cascade and their deficiency leading to thrombus formation. The coagulation cascade involves a series of steps leading to the formation of a blood clot, and deficiencies in certain factors can paradoxically lead to a prothrombotic state.

## Why the Correct Answer is Right
The correct answer, **Protein C**, when deficient, leads to an increased risk of thrombus formation. Protein C is an anticoagulant protein that, when activated, inhibits factors Va and VIIIa, thereby preventing excessive clot formation. A deficiency in protein C or its cofactor, protein S, tips the balance towards clotting.

## Why Each Wrong Option is Incorrect
- **Option A: Antithrombin III** - A deficiency in antithrombin III increases the risk of thrombosis because antithrombin III inhibits several steps in the coagulation cascade, including the inactivation of thrombin and factor Xa.
- **Option B: Protein S** - Similar to protein C, protein S acts as a cofactor for activated protein C. A deficiency in protein S also leads to a hypercoagulable state.
- **Option D: Plasminogen** - Plasminogen is involved in fibrinolysis, the process of dissolving clots. A deficiency in plasminogen can lead to decreased fibrinolytic activity, thereby increasing the risk of thrombosis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while deficiencies in anticoagulant proteins (like protein C, protein S, and antithrombin III) and fibrinolytic system components (like plasminogen) can lead to a prothrombotic state, not all coagulation factor deficiencies result in thrombosis. For instance, deficiencies in factors involved in the coagulation cascade (like factors VIII, IX, XI, and XII) are more commonly associated with bleeding disorders.

## Correct Answer: C. Protein C.