Fondaparinux is a synthetic pentasaccharide anticoagulant. Its mechanism differs from unfractionated heparin in that:

• A Fondaparinux directly inhibits thrombin without requiring antithrombin III
• B Fondaparinux selectively catalyzes AT-III inhibition of Factor Xa only, without inhibiting thrombin (Factor IIa) ✓
• C Fondaparinux directly binds and inactivates both Factor Xa and thrombin simultaneously
• D Fondaparinux activates protein C pathway rather than antithrombin III

Explanation

Unfractionated heparin (UFH) is a large molecule (5,000-30,000 Da) that binds antithrombin III via a specific pentasaccharide sequence, and the heparin-AT complex then inactivates both Factor Xa and thrombin (IIa). However, inactivation of thrombin requires the heparin chain to be long enough to bridge AT-III and thrombin simultaneously (minimum 18 saccharide units). Fondaparinux is only the synthetic pentasaccharide sequence (1,728 Da) that binds AT-III and catalyzes its inactivation of Factor Xa exclusively — the molecule is too small to bridge to thrombin. This selectivity means fondaparinux has no effect on thrombin, aPTT is not prolonged, and it cannot be reversed with protamine.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.