A 28-year-old man presents for severe abdominal pain and is diagnosed with appendicitis. He is taken for emergent appendectomy. During the procedure, the patient has massive and persistent bleeding requiring a blood transfusion. The preoperative laboratory studies showed a normal bleeding time, normal prothrombin time (PT), an INR of 1.0, and a normal platelet count. Postoperatively, when the patient is told about the complications during the surgery, he recalls that he forgot to mention that he has a family history of an unknown bleeding disorder. The postoperative laboratory tests reveal a prolonged partial thromboplastin time (PTT). Which of the following is the most likely diagnosis in this patient?

Thrombotic thrombocytopenic purpura

A 30-year-old woman presents to the emergency department in a state of confusion and disorientation that started this morning. She is accompanied by her husband who says that she has been unwell for about one week. She has been complaining of fatigue. Her husband says that this morning, she also complained that her urine was dark red in color and that there were some red spots over her legs. He did notice some changes in her level of consciousness that worsened over time and he decided to bring her in today. She does not have a significant medical history. Physical examination shows petechiae over her arms and legs. She is conscious but drowsy and disoriented and unable to answer the physician’s questions appropriately. Her temperature is 38.3°C (100.9°F), blood pressure is 160/100 mm Hg, pulse rate is 90/min, and respiratory rate is 20/min. Laboratory studies show: Hemoglobin 10 g/dL Leukocyte count 9,000/mm3 Platelet count 30,000/mm3 Bleeding time 10 min Prothrombin time 12 s Activated partial thromboplastin time 30 s D-dimer 0.4 mg/L (normal < 0.5 mg/L) Serum fibrinogen 350 mg/dL (normal 200–400 mg/dL) Serum bilirubin (indirect) 2.2 mg/dL Serum creatinine 1.5 mg/dL Serum LDH 1,010 U/L Based on her history, and her physical and laboratory findings, which of the following is the most likely pathophysiology for her presentation?

Decreased ADAMTS13 causing platelet adhesion and formation of microthrombi

GPIIb/IIIa deficiency and failure of platelet aggregation

Systemic activation of blood coagulation

E. coli-mediated endothelial damage and formation of microthrombi

A 4-year-old male is evaluated for frequent epistaxis and mucous membrane bleeding. Physical examination shows diffuse petechiae on the patient’s distal extremities. Peripheral blood smear shows an absence of platelet clumping. An ELISA binding assay reveals that platelet surfaces are deficient in GIIb/IIIa receptors. Serum platelet count is normal. Which of the following is the most likely diagnosis?

Idiopathic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura

An 8-year-old boy is brought to the pediatric emergency department by his parents with a complaint of abdominal pain and diarrhea for the past week. He states that for the past two days, he has noticed blood in his stool. His parents note that they attended a neighbor’s barbecue last weekend, but otherwise have not eaten any new foods or changed their usual diet. The patient is admitted to the hospital unit for further work-up. The provider team finds that the patient’s blood is positive for Shiga-like toxin and notes the following lab values: creatinine of 4.2 mg/dL, platelet count of 50,000/mm^3, and hemoglobin of 6.0 g/dL. Which of the following additional lab findings would be consistent with the diagnosis?

Microthrombi within glomerular vessels on kidney biopsy

Blunting of villi on ileal biopsy

Crypt abscesses and ulcers on colonic biopsy

Foamy macrophages in intestinal lamina propria on duodenal biopsy

Sickling of red blood cells on peripheral blood smear

A previously healthy 4-year-old boy is brought to the emergency department because of a 1-day history of pain and swelling of his left knee joint. He has not had any trauma to the knee. His family history is unremarkable except for a bleeding disorder in his maternal uncle. His temperature is 36.9°C (98.4°F). The left knee is erythematous, swollen, and tender; range of motion is limited. No other joints are affected. An x-ray of the knee shows an effusion but no structural abnormalities of the joint. Arthrocentesis is conducted. The synovial fluid is bloody. Further evaluation of this patient is most likely to show which of the following findings?

Prolonged partial thromboplastin time

Elevated antinuclear antibody levels

A 13-year-old girl is brought to the pediatrician due to a 4-month history of heavy vaginal bleeding during menstrual periods. She endorses episodes of bleeding gums after brushing her teeth and experienced prolonged bleeding after tonsillectomy 6 years ago. Her mother states that she bled significantly during childbirth and that the girl’s older brother has similar symptoms including easy bruising. Vitals were stable and physical exam was not revealing. Laboratory studies show: Platelet count: 72,000/mm^3 Bleeding time: 14 min Prothrombin time: 12 secs (INR = 1) Partial thromboplastin time: 40 secs Blood smear demonstrates increased megakaryocytes and enlarged platelets. Platelets do not aggregate to ristocetin. Which of the following is the most likely diagnosis?

Idiopathic thrombocytopenic purpura (ITP)

Coagulation disorders for USMLE Step 2 CK: High-Yield Pathology Lessons

Coagulation Cascade - The Clotting Symphony

Two main arms: Intrinsic (contact activation) and Extrinsic (tissue factor). They converge on the Common Pathway.
Goal: Convert fibrinogen (Factor I) to a stable fibrin clot.
Key Players: Vitamin K-dependent factors (II, VII, IX, X, Proteins C & S).

Coagulation Cascade Pathways and Related Disorders

⭐ Calcium (Factor IV) is essential for the function of multiple clotting factors. Without it, the cascade grinds to a halt. This is why citrate is used in blood collection tubes to chelate calcium and prevent clotting.

📌 Mnemonic: The extrinsic pathway is initiated by factor 7 and is measured by PT (7 letters in "Extrinsic").

Primary Hemostasis - Platelet Problems

Pathophysiology: Defective platelet plug formation, leading to mucocutaneous bleeding.
Clinical Signs: Petechiae, purpura, ecchymoses, epistaxis, gingival bleeding. Prolonged bleeding time (BT).

Platelet activation and vWF-GPIb-GPIIb/IIIa signaling

Quantitative Disorders (Thrombocytopenia)
- Immune Thrombocytopenia (ITP): Anti-GpIIb/IIIa Abs → splenic destruction. Isolated ↓ platelets. Tx: corticosteroids, IVIG.
- Thrombotic Thrombocytopenic Purpura (TTP): Deficient ADAMTS13. Pentad: fever, MAHA, thrombocytopenia, renal/neuro signs.
Qualitative Disorders (Thrombocytopathy)
- Bernard-Soulier Syndrome: Deficient GpIb → impaired adhesion. Large platelets.
- Glanzmann Thrombasthenia: Deficient GpIIb/IIIa → impaired aggregation.

⭐ In TTP, plasma exchange is the emergent treatment of choice. Do not wait for the full pentad to manifest before initiating therapy.

Secondary Hemostasis - Factor Fiascos

X-linked recessive disorders causing deep tissue bleeding & hemarthrosis.
- Hemophilia A: Factor VIII deficiency. Most common.
- Hemophilia B (Christmas Disease): Factor IX deficiency.
Labs: ↑ PTT, normal PT & platelet count.
Treatment: Desmopressin (for mild Hemophilia A), recombinant factor concentrates.
Acquired Coagulopathy:
- Vitamin K Deficiency: ↓ Factors II, VII, IX, X, Protein C/S. ↑ PT (first) and PTT. Seen in liver disease, malabsorption, warfarin use.

Coagulation Cascade: Intrinsic, Extrinsic, Common Pathways

⭐ Mixing studies differentiate factor deficiency from a factor inhibitor. PTT/PT corrects with a deficiency but remains prolonged if an inhibitor (e.g., anti-FVIII antibody) is present.

Mixed Disorders - Clot & Bleed Chaos

Disseminated Intravascular Coagulation (DIC): Widespread activation of coagulation leads to consumption of clotting factors & platelets, causing simultaneous thrombosis and hemorrhage.
- Triggers: 📌 Sepsis, Trauma, Obstetrics, Pancreatitis, Malignancy.
- Labs: ↑ PT/PTT, ↑ D-dimer, ↓ Platelets, ↓ Fibrinogen.
Thrombotic Thrombocytopenic Purpura (TTP): Autoantibodies against ADAMTS13.
- Pentad: 📌 Fever, Anemia (MAHA), Thrombocytopenia, Renal failure, Neurologic symptoms.
Hemolytic Uremic Syndrome (HUS): Endothelial damage by Shiga-like toxin (E. coli O157:H7).
- Triad: MAHA, Thrombocytopenia, Acute Kidney Injury.

Thrombotic Microangiopathy and Hemolytic Anemia (MAHA)

⭐ In TTP and HUS, coagulation studies (PT/PTT) are typically normal, distinguishing them from DIC where these are prolonged.

High‑Yield Points - ⚡ Biggest Takeaways

Hemophilia A (Factor VIII) & B (Factor IX) are X-linked disorders causing deep tissue bleeding and an isolated ↑ PTT.

Vitamin K deficiency (Factors II, VII, IX, X) and liver disease cause an ↑ in both PT and PTT.

von Willebrand disease is the most common, an autosomal dominant defect in platelet adhesion causing mucocutaneous bleeding.

DIC involves widespread clot consumption, leading to both thrombosis and hemorrhage with ↑ D-dimer and ↓ platelets.

Platelet defects: Bernard-Soulier (adhesion, GpIb) vs. Glanzmann (aggregation, GpIIb/IIIa).

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