**Question:** All of the following are parenteral Direct Thrombin Inhibitor except

A. Argatroban
B. Danaparoid
C. Hirudin
D. Lepirudin

**Correct Answer: C. Hirudin**

**Core Concept:** Direct thrombin inhibitors are a class of anticoagulant medications that work by inhibiting the activity of thrombin, a key enzyme in the coagulation cascade. This helps to prevent blood clot formation and dissolve existing clots.

**Why the Correct Answer is Right:** Hirudin is a specific inhibitor of thrombin, but it is not a parenteral drug. Hirudin is a protein derived from the saliva of the common leech (Hirudo medicinalis). While it is effective in inhibiting thrombin, Hirudin is administered through a different route, making it an exception to the list of parenteral direct thrombin inhibitors.

**Why Each Wrong Option is Incorrect:**

**A. Argatroban:** Argatroban is a direct thrombin inhibitor and is administered parenterally, making it the correct choice. Argatroban is an amidolytic inhibitor, meaning it directly binds to thrombin and inhibits its activity.

**B. Danaparoid:** Danaparoid is another direct thrombin inhibitor, also administered parenterally. It is an heparinoid, meaning it contains heparin sulfate molecules that inhibit thrombin activity.

**D. Lepirudin:** Lepirudin, also known as recombinant hirudin, is a direct thrombin inhibitor administered parenterally. It is a recombinant form of hirudin, so it shares the same mechanism of action but is administered differently.

**Clinical Pearl:** Direct thrombin inhibitors are essential medications in the management of thrombotic disorders, especially in patients with heparin-induced thrombocytopenia (HIT). They are used to prevent and treat various thromboembolic conditions, but the correct route of administration varies between the options provided.

**Explanation of the Question:** The question is testing the understanding of the different routes of administration for direct thrombin inhibitors and their mechanism of action. It also highlights the differences between recombinant and natural forms of direct thrombin inhibitors.