A patient receiving therapeutic unfractionated heparin fails to achieve target aPTT prolongation despite escalating doses. Anti-Xa assay confirms low heparin bioactivity. The most likely explanation is:

• A Factor V Leiden mutation
• B Lupus anticoagulant
• C Dysfibrinogenemia
• D Antithrombin III deficiency ✓

Explanation

Heparin acts as a catalyst by binding antithrombin III and accelerating its inhibition of thrombin and factor Xa roughly a thousandfold. Congenital or acquired antithrombin deficiency produces heparin resistance, requiring higher doses or antithrombin concentrate. Lupus anticoagulant prolongs aPTT but does not blunt heparin responsiveness, and factor V Leiden affects activated protein C cleavage.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.