Hemophilia A vs B is one of those Step-style “you either know it instantly or you don’t” distinctions. The good news: you can lock it in with a simple visual + a couple of high-yield anchors (factor number, inheritance, lab pattern, and treatment).
The 10-second mnemonic (shareable)
“A = Eight”
- Hemophilia A = Factor VIII deficiency
- Think: A looks like it has two “legs” → 8 (a quick visual cue)
“B = Nine”
- Hemophilia B = Factor IX deficiency
- Think: B looks like a “9” (big loop + tail)
One-liner:
Hemophilia A = Factor VIII, Hemophilia B = Factor IX — both cause deep bleeding with isolated ↑PTT.
Quick visual: “Ate vs Bee”
Use this tiny mental picture when you see a stem:
| Condition | Visual cue | Factor deficiency |
|---|---|---|
| Hemophilia A | “Ate” → “Eight” | Factor VIII |
| Hemophilia B | Bee shaped like “9” | Factor IX |
High-yield Step facts (what question writers love)
Shared features (A and B)
- Inheritance: X-linked recessive
- Typically males affected, females carriers
- Bleeding pattern: Deep tissue bleeding
- Hemarthroses (joint bleeding)
- Muscle hematomas
- Delayed bleeding after procedures/trauma
- Labs: Isolated prolonged PTT
- PT normal
- Platelet count and bleeding time usually normal
- Pathway: Intrinsic pathway defects
- Classic hook: Intrinsic = “PTT”
The “confirm it” lab trick: Mixing study
When Step questions want you to separate factor deficiency from an inhibitor:
- Mixing study corrects PTT → suggests factor deficiency (e.g., Hemophilia A/B)
- Mixing study does NOT correct PTT → suggests inhibitor (e.g., acquired factor VIII inhibitor, lupus anticoagulant)
Treatment pearls (commonly tested)
Hemophilia A (Factor VIII deficiency)
- Desmopressin (DDAVP) for mild disease
- Mechanism: increases release of factor VIII and vWF from endothelial cells
- Recombinant factor VIII for more significant bleeding or procedures
- Step pitfall: acquired factor VIII inhibitors can cause refractory bleeding and PTT not correcting on mixing
Hemophilia B (Factor IX deficiency)
- Recombinant factor IX replacement
Transfusion caution: Factor concentrates are preferred; plasma products are less targeted and carry more volume/infectious risk.
Classic vignette snapshots (burn these in)
- Teen boy + recurrent knee swelling after sports + ↑PTT + normal platelets → Hemophilia (A or B)
- Bleeding after circumcision + family history through maternal line → XLR hemophilia
- Postpartum woman with new severe bleeding + isolated ↑PTT that doesn’t correct → acquired factor VIII inhibitor (not congenital hemophilia)
Ultra-fast recap (what you should be able to blurt out)
- A = VIII, B = IX
- Both are X-linked recessive, cause hemarthroses, and show isolated ↑PTT
- Mixing study corrects in deficiencies
- Treat A with DDAVP (mild) or factor VIII; treat B with factor IX