A 72-year-old man with a history of severe rheumatoid arthritis is brought to the emergenc… — USMLE Step 3

A 72-year-old man with a history of severe rheumatoid arthritis is brought to the emergency department due to progressive swelling and severe pain in his right thigh over the past 48 hours. He denies any recent trauma, surgery, or changes to his medications. His current medications include methotrexate and prednisone. Vital signs show a blood pressure of 105/65 mm Hg and a heart rate of 112/min. Physical examination reveals a massive, tense, ecchymotic swelling extending from the right groin to the knee. Laboratory studies show:

Test	Result	Reference Range
Hemoglobin	7.2 g/dL	13.5-17.5 g/dL
Platelet count	210,000/mm³	150,000-400,000/mm³
Prothrombin time (PT)	12 sec	11-15 sec
Activated partial thromboplastin time (aPTT)	94 sec	25-40 sec

A mixing study is performed using a 1:1 mixture of the patient's plasma and normal pooled plasma. Immediately after mixing, the aPTT is 34 seconds. After incubating the mixture at $37^\circ\text{C}$ for 2 hours, the aPTT is 82 seconds.

Which of the following describes the mechanism of action of the most appropriate immediate hemostatic pharmacotherapy for this patient?

Complexing with exposed tissue factor to independently catalyze the conversion of factor X to factor XaCevap
B
Direct replacement of the deficient glycoprotein to competitively overcome autoantibody neutralization
C
Stimulation of endothelial Weibel-Palade bodies to release stored endogenous coagulation factors
D
Bispecific bridging of activated factor IX and factor X to restore the intrinsic tenase complex
E
Provision of a concentrated cryoprecipitate containing high levels of fibrinogen and factor VIII

Cevap

Recombinant factor VIIa (rFVIIa), which complexes with exposed tissue factor to independently catalyze the conversion of factor X to factor Xa.

The correct answer identifies the mechanism of recombinant factor VIIa (rFVIIa). This patient's presentation of spontaneous severe bleeding in the setting of an autoimmune disease (rheumatoid arthritis), combined with an isolated prolonged aPTT that corrects immediately but prolongs after incubation, is diagnostic of acquired hemophilia A (Factor VIII autoantibodies). Because the inhibitor is time- and temperature-dependent, the autoantibodies will rapidly neutralize standard factor VIII replacement. Therefore, acute severe bleeding must be managed with a 'bypassing agent' such as rFVIIa or activated prothrombin complex concentrate (aPCC). rFVIIa works by complexing with tissue factor at the site of vascular injury to directly catalyze the conversion of factor X to factor Xa, bypassing the blocked intrinsic tenase complex (Factor IXa + Factor VIIIa).

Adım Adım Çözüm

Analyze the basic coagulation panel.

The patient has an isolated, markedly prolonged aPTT with a normal PT and normal platelet count.

This isolates the defect to the intrinsic coagulation pathway (factors VIII, IX, XI, or XII).

Interpret the mixing study results.

The aPTT corrects immediately upon mixing but becomes prolonged again after a 2-hour incubation at

37^\circ\text{C}

This is the hallmark of a time- and temperature-dependent inhibitor, specifically an autoantibody against Factor VIII, confirming a diagnosis of acquired hemophilia A.

Determine the optimal acute management for severe bleeding in this condition.

Standard factor replacement will be rapidly neutralized. A bypassing agent is required.

The patient has life-threatening hemorrhage (massive thigh hematoma, hemoglobin 7.2 g/dL) requiring immediate bypass of the inhibited intrinsic tenase complex.

Identify the mechanism of action of the bypassing agent.

Recombinant factor VIIa (rFVIIa) or activated prothrombin complex concentrate (aPCC) must be used.

rFVIIa forms a complex with tissue factor at the site of vascular injury to directly activate factor X, bypassing the need for factor VIIIa altogether.

Anahtar Kavram

Acquired hemophilia A is characterized by an isolated prolonged aPTT that corrects on immediate mixing but prolongs after incubation; acute severe bleeding is treated with bypassing agents (rFVIIa or aPCC) which activate factor X independently of the intrinsic tenase complex.

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