Hematopathology — MCQs

Patients with lymphoproliferative disorders undergo cytogenetic analysis. A subset of these cases shows the BCR-ABL fusion gene from the reciprocal translocation t(9;22)(q34;11), resulting in increased tyrosine kinase activity. Which of the following conditions is most likely to present with this gene?

Q470Easy

What is the major source of Von Willebrand factor (vWF)?

Hematopathology Indian Medical PG Practice Questions and MCQs

Question 461: All of the following are seen in paroxysmal nocturnal hemoglobinuria (PNH) except?

A. Arterial thrombosis (Correct Answer)
B. Venous thrombosis
C. Aplastic anemia
D. Hemolytic anemia

Explanation: Paroxysmal Nocturnal Hemoglobinuria (PNH) is an acquired clonal stem cell disorder caused by a somatic mutation in the **PIGA gene**, leading to a deficiency of GPI-anchored proteins, specifically **CD55 (DAF)** and **CD59 (MIRL)** [2]. These proteins normally protect cells from complement-mediated lysis. **1. Why "Arterial Thrombosis" is the Correct Answer:** While PNH is a highly prothrombotic state, it is classically associated with **Venous Thrombosis**, particularly in unusual sites (e.g., hepatic, portal, or cerebral veins). While arterial events can rarely occur, the hallmark and most common cause of death in PNH is venous thromboembolism (Budd-Chiari syndrome) [1]. In the context of NEET-PG questions, PNH is the classic "exception" where venous thrombosis is far more characteristic than arterial. **2. Analysis of Other Options:** * **Venous Thrombosis:** The most common complication and leading cause of mortality [1]. Hemolysis releases free hemoglobin and inflammatory mediators that trigger a hypercoagulable state. * **Aplastic Anemia:** PNH and Aplastic Anemia (AA) are closely linked. PNH clones often arise in the setting of an empty bone marrow (AA/PNH syndrome), and patients may transition between these conditions [3]. * **Hemolytic Anemia:** PNH is characterized by chronic intravascular hemolysis due to the absence of CD59, which allows the Complement Membrane Attack Complex (MAC) to lyse red cells [1]. **Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Flow cytometry (showing absence of CD55/CD59 on RBCs and WBCs). * **FLAER (Fluorescent Proaerolysin) test:** Highly sensitive for detecting GPI-deficient clones. * **Classic Triad:** Hemolytic anemia, pancytopenia (bone marrow failure), and venous thrombosis. * **Treatment:** Eculizumab (Monoclonal antibody against C5). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 650-651. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 601-602. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 595-596.

Question 462: In Beta-thalassemia trait, which of the following is characteristically elevated?

A. Hemoglobin F (HbF)
B. Hemoglobin A2 (HbA2) (Correct Answer)
C. Microcytosis
D. Severe anemia

Explanation: ### Explanation **Correct Answer: B. Hemoglobin A2 (HbA2)** **Underlying Concept:** Beta-thalassemia trait (Beta-thalassemia minor) is caused by a heterozygous mutation in the $\beta$-globin gene, leading to a partial deficiency of $\beta$-globin chains [1]. To compensate for the lack of $\beta$-chains, there is a compensatory increase in the synthesis of $\delta$-globin chains. These $\delta$-chains combine with $\alpha$-chains to form **Hemoglobin A2 ($\alpha_2\delta_2$)**. In a normal adult, HbA2 is $<3.5\%$; however, in Beta-thalassemia trait, it characteristically rises to **$>3.5\%$ (usually 4-8%)**. This is the gold-standard diagnostic marker for the carrier state. **Analysis of Incorrect Options:** * **A. Hemoglobin F (HbF):** While HbF ($\alpha_2\gamma_2$) can be slightly elevated in the trait (1-5%), it is not as consistently or characteristically elevated as HbA2. Massive elevations of HbF ($>90\%$) are seen in Beta-thalassemia *major* [2]. * **C. Microcytosis:** While microcytosis (low MCV) is a hallmark feature of Beta-thalassemia trait, the question asks what is **elevated** [1]. Microcytosis refers to a *decrease* in cell size. * **D. Severe anemia:** Patients with the trait typically have mild anemia or are asymptomatic [1]. Severe, transfusion-dependent anemia is characteristic of Beta-thalassemia major (Cooley’s Anemia) [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Mentzer Index:** (MCV/RBC count) $<13$ suggests Thalassemia trait; $>13$ suggests Iron Deficiency Anemia (IDA). * **RBC Count:** In Beta-thalassemia trait, the RBC count is often **paradoxically high** despite low hemoglobin. * **Peripheral Smear:** Shows microcytic hypochromic cells with **target cells** and basophilic stippling. * **Diagnosis:** Hb Electrophoresis or HPLC is used to quantify the HbA2 level. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 649-650. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, p. 648.

Question 463: Beta Thalassemia is best diagnosed by?

A. NESTROFT Test
B. HbA1c estimation
C. Target cells in peripheral smear
D. Hb electrophoresis (Correct Answer)

Explanation: **Explanation:** **Beta Thalassemia** is a quantitative hemoglobinopathy characterized by reduced or absent synthesis of beta-globin chains [1]. The diagnosis relies on demonstrating an abnormal distribution of hemoglobin types [2]. **Why Hb Electrophoresis is the Correct Answer:** Hb electrophoresis (or HPLC) is the **gold standard** for diagnosing Beta Thalassemia trait. In a normal adult, HbA ($\alpha_2\beta_2$) is the predominant form. In Beta Thalassemia minor, the body compensates for the lack of beta chains by increasing the production of delta chains, leading to a characteristic **elevation of HbA2 (>3.5%)**. It may also show an increase in HbF. This quantitative analysis allows for a definitive diagnosis [2]. **Analysis of Incorrect Options:** * **NESTROFT Test:** This is a **screening test** (Naked Eye Single Tube Red Cell Osmotic Fragility Test) used in mass screenings due to its high sensitivity but low specificity. It does not provide a definitive diagnosis. * **HbA1c estimation:** This is used to monitor long-term glycemic control in Diabetes Mellitus. In fact, Thalassemia can falsely lower HbA1c levels due to increased red cell turnover. * **Target cells in peripheral smear:** While target cells (codocytes) are a classic morphological finding in Thalassemia, they are **non-specific** and can also be seen in liver disease, post-splenectomy, and Iron Deficiency Anemia (IDA) [1]. **Clinical Pearls for NEET-PG:** * **Mentzer Index:** (MCV/RBC count) <13 suggests Thalassemia; >13 suggests IDA. * **HPLC (High-Performance Liquid Chromatography):** Currently preferred over electrophoresis in modern labs for its precision in quantifying HbA2. * **Iron Studies:** Always rule out Iron Deficiency before interpreting HbA2, as IDA can falsely lower HbA2 levels, potentially masking a Thalassemia trait. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 600-601. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, p. 650.

Question 464: Hereditary spherocytosis is characterized by what type of abnormality?

A. Membrane skeleton abnormalities (Correct Answer)
B. Microtubule defect
C. Accumulation of intermediate filament
D. None of the above

Explanation: **Explanation:** **Hereditary Spherocytosis (HS)** is an autosomal dominant disorder characterized by an intrinsic defect in the red blood cell (RBC) membrane [1]. **1. Why Option A is Correct:** The primary pathology in HS involves mutations in genes encoding proteins of the **RBC membrane skeleton** [1]. The most common proteins involved are **Ankyrin** (most common), **Band 3**, **Spectrin**, and **Protein 4.2** [1]. These proteins normally tether the lipid bilayer to the underlying cytoskeleton. A defect leads to a loss of membrane surface area (blebbing), forcing the cell to assume the smallest possible volume for its contents—a **sphere** [1]. These spherocytes are rigid, lack central pallor, and are prematurely destroyed in the splenic sinusoids (extravascular hemolysis) [1]. **2. Why Other Options are Incorrect:** * **Option B (Microtubule defect):** This is characteristic of conditions like **Chediak-Higashi syndrome** (defective vesicle trafficking) or **Kartagener syndrome** (dynein arm defect in cilia), not RBC membrane disorders. * **Option C (Intermediate filament accumulation):** This is seen in conditions like **Mallory-Denk bodies** in alcoholic liver disease (pre-keratin) or neurodegenerative diseases, but it plays no role in the pathogenesis of HS. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Test:** Eosin-5-maleimide (EMA) binding test (Flow cytometry). * **Screening Test:** Osmotic Fragility Test (increased fragility) [2]. * **Peripheral Smear:** Spherocytes (small, dark RBCs without central pallor) and increased reticulocytes [2]. * **Lab Findings:** Increased **MCHC** (highly characteristic) and indirect hyperbilirubinemia. * **Complications:** Pigment gallstones (cholelithiasis) and aplastic crisis (associated with Parvovirus B19) [2]. * **Treatment:** Splenectomy is the definitive treatment to stop hemolysis (though it does not correct the underlying membrane defect) [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 640-641. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 597-598.

Question 465: Drepanocytes are seen in which of the following conditions?

A. Megaloblastic anemia
B. Severe iron deficiency anemia
C. Sickle cell anemia (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Drepanocytes** is the medical term for **sickle cells**. These are elongated, crescent-shaped red blood cells with pointed ends, resembling a sickle or a scimitar. **1. Why Sickle Cell Anemia (SCA) is correct:** The underlying pathology in SCA is a point mutation in the $\beta$-globin gene (glutamic acid replaced by valine at the 6th position) [2]. Under conditions of low oxygen tension (hypoxia), acidosis, or dehydration, the abnormal Hemoglobin S (HbS) polymerizes into long, stiff insoluble fibers [1]. these fibers distort the red cell membrane, transforming the normal biconcave disc into a **drepanocyte** [1], [2]. **2. Why other options are incorrect:** * **Megaloblastic Anemia:** Characterized by **Macro-ovalocytes** (large, oval RBCs) and hypersegmented neutrophils due to impaired DNA synthesis (Vitamin B12/Folate deficiency). * **Severe Iron Deficiency Anemia:** Characterized by **Microcytic Hypochromic** cells, pencil cells (elliptocytes), and occasionally target cells, but never drepanocytes. **3. NEET-PG High-Yield Pearls:** * **Irreversible vs. Reversible:** Sickle cells can initially revert to normal shape upon re-oxygenation, but repeated cycles of sickling lead to membrane damage, resulting in **irreversibly sickled cells** [1]. * **Other Morphologies in SCA:** Look for **Howell-Jolly bodies** on a peripheral smear, which indicate functional asplenia (autosplenectomy) [3]. * **Diagnostic Test:** While drepanocytes are seen on a peripheral smear, the gold standard for diagnosis is **Hemoglobin Electrophoresis** or HPLC. * **Inducing Sickling:** In vitro, sickling can be induced using reducing agents like **Sodium metabisulfite**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 652-654. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 598-599. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 645-646.

Question 466: Rate of sickling in sickle cell anemia is directly proportional to all of the following EXCEPT:

A. Raised HbS
B. Raised HbC
C. Raised HbF (Correct Answer)
D. Hb deoxygenation

Explanation: ### Explanation The rate and severity of sickling in Sickle Cell Anemia (HbSS) depend on the polymerization of deoxygenated hemoglobin S (HbS). The correct answer is **Raised HbF** because it is a potent **inhibitor** of sickling, making the rate of sickling *inversely* proportional to its concentration. #### Why Raised HbF is the Correct Answer: Fetal Hemoglobin (HbF) does not participate in the polymerization of HbS. More importantly, it prevents the formation of HbS polymers even when deoxygenated. This is why newborns with Sickle Cell Anemia do not show symptoms until 6 months of age, as HbF levels naturally decline and are replaced by HbS. #### Analysis of Other Options: * **Raised HbS (Option A):** The concentration of HbS is the primary driver of polymerization. Higher concentrations lead to faster and more extensive sickling [1]. * **Raised HbC (Option B):** HbC (Hemoglobin C) has a higher tendency to aggregate with HbS than HbA does. In HbSC disease, HbC increases the intracellular concentration of HbS by causing cellular dehydration, thereby **increasing** the rate of sickling [1]. * **Hb Deoxygenation (Option D):** Sickling occurs specifically when HbS is in the deoxygenated state (T-state). Factors that promote deoxygenation (acidosis, increased 2,3-BPG, high altitude) directly increase the rate of sickling. #### NEET-PG High-Yield Pearls: * **Hydroxyurea:** The mainstay of treatment for Sickle Cell Anemia because it pharmacologically **increases HbF levels**, reducing the frequency of painful crises. * **MCHC:** The rate of sickling is highly sensitive to the Mean Corpuscular Hemoglobin Concentration [1]. Dehydration increases MCHC and worsens sickling. * **Protective Factor:** HbA (in Sickle Cell Trait) is a better inhibitor of sickling than HbC, but HbF is the most potent inhibitor of all. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 643-644.

Question 467: Progressive transformation of germinal centers (PTGC) is a precursor lesion of which of the following?

A. Hodgkin lymphoma, nodular sclerosis
B. Hodgkin lymphoma, mixed cellularity
C. Hodgkin lymphoma, lymphocytic predominant (Correct Answer)
D. Peripheral T cell lymphoma

Explanation: **Explanation:** **Progressive Transformation of Germinal Centers (PTGC)** is a reactive condition characterized by the enlargement of germinal centers due to an influx of mantle zone B-cells into the germinal center, causing it to become fragmented and "transformed." **Why Option C is correct:** PTGC is closely associated with **Nodular Lymphocyte Predominant Hodgkin Lymphoma (NLPHL)**. While PTGC is not a direct neoplastic precursor in the traditional sense, it is frequently found in lymph nodes concurrently with NLPHL or may precede/follow the diagnosis. Both conditions share a common biological background involving the expansion of the follicular meshwork and the presence of B-cells. In NLPHL, the characteristic "Popcorn cells" (LP cells) are found within these expanded nodules. **Why other options are incorrect:** * **Options A & B:** Nodular Sclerosis and Mixed Cellularity are subtypes of **Classical Hodgkin Lymphoma (cHL)**. These are characterized by Reed-Sternberg (RS) cells that are CD15+ and CD30+ [1]. NLPHL (associated with PTGC) is distinct because its cells are CD20+ and CD45+, and it lacks the typical RS cells of cHL. * **Option D:** Peripheral T-cell lymphoma involves the malignant proliferation of mature T-cells and does not involve the germinal center B-cell transformation seen in PTGC. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** PTGC presents as large, dark nodules (3–5 times the size of normal follicles) within a background of follicular hyperplasia. * **Clinical Presentation:** Usually presents as asymptomatic, localized lymphadenopathy (often cervical) in young males. * **Key Distinction:** Unlike NLPHL, PTGC is a **benign, reactive process** and does not require aggressive treatment, though it warrants close follow-up due to the risk of associated NLPHL. * **Immunophenotype of NLPHL:** CD20+, CD45+, BCL6+, but **CD15- and CD30-** (opposite of Classical HL). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, pp. 614-616.

Question 468: Which of the following is seen in Disseminated Intravascular Coagulation (DIC)?

A. Normal APTT
B. Increased PT (Correct Answer)
C. Increased factor VIII
D. None of the above

Explanation: **Explanation:** Disseminated Intravascular Coagulation (DIC) is a thrombohemorrhagic disorder characterized by the systemic activation of the coagulation cascade, leading to widespread microvascular thrombosis and the subsequent **consumption of clotting factors and platelets.** [1], [2] **1. Why Option B is Correct:** In DIC, there is massive consumption of both the extrinsic and intrinsic pathway factors. Prothrombin Time (PT) measures the extrinsic and common pathways (Factors VII, X, V, II, and Fibrinogen). As these factors are depleted during the formation of widespread microthrombi, the **PT becomes prolonged (increased).** [1] PT is often the first to be affected and is a sensitive indicator of the severity of consumption. **2. Why Other Options are Incorrect:** * **Option A (Normal APTT):** This is incorrect because the Activated Partial Thromboplastin Time (APTT) measures the intrinsic pathway. Just like PT, APTT is **prolonged** in DIC due to the consumption of factors VIII, IX, XI, and XII. [1] * **Option C (Increased Factor VIII):** This is incorrect. Factor VIII is a consumable clotting factor. In DIC, levels of **Factor VIII and Factor V are significantly decreased.** [1] (Note: Factor VIII is also an acute-phase reactant, but in the context of active DIC, consumption outweighs production). **NEET-PG High-Yield Pearls for DIC:** * **Best Screening Test:** Platelet count (Thrombocytopenia is almost universal). [1] * **Most Specific Test:** D-dimer (indicates fibrin degradation). [2] * **Peripheral Smear:** Presence of **Schistocytes** (fragmented RBCs) due to microangiopathic hemolytic anemia (MAHA). [1] * **Fibrinogen Levels:** Decreased (Hypofibrinogenemia). [1] * **Common Triggers:** Sepsis (most common), Obstetric complications (Abruptio placentae), and Malignancy (APML - M3). [3] **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 624-626. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 151-152. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 672-673.

Question 469: Patients with lymphoproliferative disorders undergo cytogenetic analysis. A subset of these cases shows the BCR-ABL fusion gene from the reciprocal translocation t(9;22)(q34;11), resulting in increased tyrosine kinase activity. Which of the following conditions is most likely to present with this gene?

A. Acute promyelocytic leukemia
B. Chronic myelogenous leukemia (Correct Answer)
C. Follicular lymphoma
D. Hodgkin lymphoma, lymphocyte depletion type

Explanation: **Explanation:** The correct answer is **Chronic Myelogenous Leukemia (CML)** [1]. The question describes the **Philadelphia chromosome (Ph)**, which is the hallmark of CML [2]. This occurs due to a reciprocal translocation between chromosomes 9 and 22, denoted as **t(9;22)(q34;q11)**. This translocation fuses the *ABL1* gene (from chromosome 9) with the *BCR* gene (on chromosome 22), creating the **BCR-ABL1 fusion gene** [4]. This gene encodes a chimeric protein with constitutive **tyrosine kinase activity**, which drives uncontrolled proliferation of the myeloid lineage [1]. **Analysis of Incorrect Options:** * **Acute Promyelocytic Leukemia (APL):** Characterized by **t(15;17)**, involving the *PML-RARA* fusion gene. It is treated with All-Trans Retinoic Acid (ATRA). * **Follicular Lymphoma:** Associated with **t(14;18)**, which leads to the overexpression of the **BCL-2** anti-apoptotic protein. * **Hodgkin Lymphoma (Lymphocyte Depletion):** This is a morphological diagnosis and is not defined by a specific translocation like t(9;22). It is often associated with EBV infection and a poor prognosis. **High-Yield Clinical Pearls for NEET-PG:** * **Philadelphia Chromosome:** Present in >95% of CML cases [1]. It is also found in 25-30% of adult B-ALL (associated with poor prognosis) and 2-5% of pediatric B-ALL. * **Protein Products:** CML typically produces a **p210** protein, while Ph+ ALL often produces a **p190** protein. * **Targeted Therapy:** **Imatinib** (a tyrosine kinase inhibitor) specifically targets the BCR-ABL protein and is the first-line treatment for CML [3]. * **Diagnosis:** Gold standard is FISH or RT-PCR to detect the fusion gene/transcript. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, p. 624. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 225-226. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 611-612. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, pp. 624-625.

Question 470: What is the major source of Von Willebrand factor (vWF)?

A. Erythrocytes
B. Neutrophils
C. Endothelial cells (Correct Answer)
D. Monocytes

Explanation: **Explanation:** Von Willebrand Factor (vWF) is a large multimeric glycoprotein essential for primary hemostasis. It acts as a molecular bridge between the subendothelial collagen and platelets (via the GpIb receptor) and serves as a carrier protein for Factor VIII [1]. **Why Endothelial Cells are the correct answer:** The two primary sites of vWF synthesis and storage are **vascular endothelial cells** and **megakaryocytes**. [1] * In endothelial cells, vWF is stored in specialized rod-shaped organelles called **Weibel-Palade bodies**. This represents the major source of circulating vWF [1]. * In megakaryocytes (and subsequently platelets), it is stored in the **$̑$-granules** [1], [3]. **Why other options are incorrect:** * **A. Erythrocytes:** Red blood cells do not synthesize or store clotting factors; their primary role is oxygen transport. * **B. Neutrophils:** While they contain granules (primary/secondary), they do not produce vWF. * **D. Monocytes:** These are phagocytic cells of the immune system and are not involved in the synthesis of vWF. **High-Yield Clinical Pearls for NEET-PG:** 1. **Weibel-Palade Bodies:** These contain both vWF and **P-selectin** (an adhesion molecule). 2. **vWF Function:** It is required for **platelet adhesion** (not aggregation) [2]. Deficiency leads to Von Willebrand Disease (vWD), the most common inherited bleeding disorder. 3. **Ristocetin Cofactor Assay:** This is the gold standard test for vWF function; Ristocetin induces vWF-mediated platelet agglutination. 4. **Factor VIII Stability:** vWF stabilizes Factor VIII; therefore, in severe vWD, the aPTT may be prolonged due to secondary Factor VIII deficiency [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Red Blood Cell and Bleeding Disorders, pp. 669-670. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Hemodynamic Disorders, Thromboembolic Disease, and Shock, p. 128. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 581-582.

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