General Pathology — MCQs

General Pathology Indian Medical PG Practice Questions and MCQs

Question 371: The ladder pattern observed in DNA electrophoresis during apoptosis is caused by the action of which enzyme?

A. Endonuclease (Correct Answer)
B. Transglutaminase
C. DNAse
D. Caspase

Explanation: **Explanation:** The hallmark of apoptosis is the activation of **endonucleases**, specifically **Caspase-Activated DNase (CAD)**. These enzymes cleave the cell’s DNA at internucleosomal linker regions. Since DNA is wrapped around histones in units of approximately 180–200 base pairs, this cleavage results in DNA fragments of varying lengths that are multiples of 180–200 bp (e.g., 200, 400, 600 bp). When these fragments are separated via gel electrophoresis, they create a characteristic **"ladder pattern."** **Analysis of Options:** * **Endonuclease (Correct):** Specifically, Ca²⁺ and Mg²⁺ dependent endonucleases cleave the linker DNA, producing the laddering effect pathognomonic for apoptosis. * **Transglutaminase:** This enzyme is involved in cross-linking cytoplasmic proteins during apoptosis to form apoptotic bodies, preventing the leakage of cellular contents. It does not cleave DNA. * **DNAse:** While DNase I and II can degrade DNA, the term is too general. In the context of the specific internucleosomal cleavage of apoptosis, "Endonuclease" is the more precise biochemical descriptor used in standard pathology texts (Robbins). * **Caspase:** These are proteases (cysteine-aspartic proteases) that initiate and execute apoptosis [1]. While they *activate* the endonuclease (by cleaving its inhibitor, ICAD), they do not directly cut the DNA [1]. **High-Yield Pearls for NEET-PG:** * **Apoptosis vs. Necrosis:** DNA laddering is specific to **Apoptosis**. In **Necrosis**, DNA degradation is random, resulting in a diffuse **"smear pattern"** on electrophoresis. * **Annexin V:** A marker used to detect apoptosis via flow cytometry; it binds to Phosphatidylserine, which flips to the outer leaflet of the plasma membrane [1]. * **Councilman Bodies:** Eosinophilic apoptotic hepatocytes seen in Viral Hepatitis. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 63-69.

Question 372: Which chromosome is involved in Down's Syndrome?

A. 21 (Correct Answer)
B. 22
C. 13
D. X chromosome

Explanation: **Explanation:** **Correct Answer: A (Chromosome 21)** Down’s Syndrome is the most common chromosomal disorder and a major cause of intellectual disability [1], [2]. It is characterized by **Trisomy 21**, meaning the individual has three copies of chromosome 21 instead of the usual two [1], [5]. In 95% of cases, this is caused by **meiotic non-disjunction**, which is strongly associated with advanced maternal age [1], [5]. Other mechanisms include Robertsonian translocation (4%) and mosaicism (1%) [2]. **Analysis of Incorrect Options:** * **Option B (Chromosome 22):** Abnormalities here are associated with **DiGeorge Syndrome** (22q11.2 deletion) or Chronic Myeloid Leukemia (Philadelphia chromosome, t(9;22)). * **Option C (Chromosome 13):** Trisomy 13 causes **Patau Syndrome**, characterized by midline defects like cleft lip/palate, holoprosencephaly, and polydactyly [2]. * **Option D (X Chromosome):** Disorders involving the X chromosome include **Turner Syndrome** (45,XO) [1] or **Klinefelter Syndrome** (47,XXY) [5]. **High-Yield Clinical Pearls for NEET-PG:** * **Cardiac:** The most common congenital heart defect is the **Atrioventricular Septal Defect (Endocardial cushion defect)**. * **Gastrointestinal:** Associated with **Duodenal atresia** ("Double bubble" sign) and Hirschsprung disease. * **Hematology:** Increased risk of **AMKL (M7)** in children <3 years and **ALL** in children >3 years. * **Neurology:** Early-onset **Alzheimer’s disease** (due to the APP gene located on chromosome 21) [3], [4]. * **Screening:** First-trimester markers include increased Nuchal Translucency and decreased PAPP-A. **References:** [1] 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. 40-41. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 171-172. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1290-1292. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Manifestations Of Central And Peripheral Nervous System Disease, pp. 720-721. [5] 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. 92-93.

Question 373: Which of the following conditions is not X-linked recessive?

A. Fragile X syndrome
B. Duchenne muscular dystrophy
C. Diabetes insipidus
D. Spinal muscular atrophy (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Spinal muscular atrophy** **Why Spinal Muscular Atrophy (SMA) is the correct answer:** Spinal muscular atrophy is an **Autosomal Recessive (AR)** disorder [1], [2], not X-linked. It is caused by a genetic defect in the **SMN1 (Survival Motor Neuron 1)** gene located on chromosome **5q13**. The disease is characterized by the degeneration of alpha motor neurons in the anterior horn of the spinal cord, leading to progressive muscle wasting and weakness [1]. **Analysis of Incorrect Options:** * **Fragile X Syndrome:** This is an **X-linked Dominant** condition (though historically grouped with X-linked disorders in many competitive exams due to its inheritance pattern involving the X chromosome). It is caused by a CGG trinucleotide repeat expansion in the *FMR1* gene [5]. * **Duchenne Muscular Dystrophy (DMD):** This is a classic **X-linked Recessive** disorder caused by a mutation in the *Dystrophin* gene (the largest known human gene). It primarily affects males, leading to pseudohypertrophy of calves and Gower’s sign. * **Diabetes Insipidus (Nephrogenic):** While there are multiple causes, the **hereditary nephrogenic** form is most commonly inherited in an **X-linked Recessive** pattern (mutations in the *V2 receptor* gene) [3]. **NEET-PG High-Yield Pearls:** 1. **SMA Type 1 (Werdnig-Hoffmann Disease):** The most severe form, presenting as a "floppy infant" at birth or within 6 months [4]. 2. **X-linked Recessive Mnemonic:** "**C**an't **G**o **H**ome **D**uring **M**idnight **F**unctions" (**C**hronic Granulomatous Disease, **G**6PD deficiency, **H**emophilia A/B, **D**MD/Becker, **M**enkes, **F**abry). 3. **Chromosome 5:** Remember SMA is associated with **5q**, whereas Cri-du-chat syndrome is associated with **5p** deletion. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1247-1248. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 150-151. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 151. [4] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Manifestations Of Central And Peripheral Nervous System Disease, pp. 731-732. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 177.

Question 374: Maximum damage to DNA is caused by which type of radiation?

A. alpha rays (Correct Answer)
B. beta rays
C. gamma rays
D. X-rays and UV rays

Explanation: The correct answer is **alpha rays**. [1] ### **Explanation** The biological damage caused by radiation is determined by its **Linear Energy Transfer (LET)**. LET refers to the amount of energy a particle or ray transfers to the tissue per unit of distance traveled. [1] 1. **Why Alpha Rays are Correct:** Alpha particles are heavy, positively charged particles (helium nuclei). Due to their large mass and charge, they have **high LET**. As they travel through tissue, they collide frequently with molecules, releasing a massive amount of energy over a very short distance. [1] This results in dense ionization, causing **direct, double-stranded DNA breaks** that are difficult for the cell to repair, leading to maximum localized damage. [2] 2. **Why Other Options are Incorrect:** * **Beta Rays:** These are high-speed electrons. They are much smaller and lighter than alpha particles, resulting in **lower LET** and less dense ionization. [1] * **Gamma Rays and X-rays:** These are forms of electromagnetic radiation (photons). They have **very low LET** and are highly penetrating. [1] They cause damage primarily through the production of free radicals (indirect action) rather than direct physical hits to the DNA backbone. [2] * **UV Rays:** These are non-ionizing radiation. They cause specific damage (pyrimidine/thymine dimers) but lack the penetrative power and ionizing energy to cause the extensive double-stranded destruction seen with alpha particles. ### **High-Yield Clinical Pearls for NEET-PG** * **Relative Biological Effectiveness (RBE):** High-LET radiation (Alpha, Neutrons) has a higher RBE than low-LET radiation (X-rays, Gamma). * **Direct vs. Indirect Action:** High-LET radiation causes **Direct damage** (hits DNA); Low-LET radiation causes **Indirect damage** (via Radiolysis of water and Free Radicals). [2] * **Most Sensitive Phase:** Cells are most sensitive to radiation in the **G2 and M phases** of the cell cycle. * **Most Sensitive Tissue:** Lymphocytes and hematopoietic cells (Law of Bergonie and Tribondeau). **References:** [1] 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. 111-112. [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. 101-102.

Question 375: What substance is commonly used for tissue preservation?

A. Ethyl alcohol
B. Formalin (Correct Answer)
C. Sodium Chloride
D. Normal Saline

Explanation: **Explanation:** **1. Why Formalin is the Correct Answer:** Formalin (specifically **10% Neutral Buffered Formalin**) is the gold standard fixative used in histopathology. It works by creating **cross-links between proteins** (forming methylene bridges), which terminates biochemical reactions, prevents autolysis (self-digestion by enzymes), and inhibits putrefaction (bacterial decay). This preserves the tissue architecture in a state as close to life as possible, allowing for thin sectioning and staining. **2. Analysis of Incorrect Options:** * **Ethyl Alcohol:** While used as a fixative for cytology smears (e.g., Pap smears), it is a dehydrating agent. Using it for primary tissue preservation causes significant tissue shrinkage and hardening, making it unsuitable for routine surgical pathology. * **Sodium Chloride / Normal Saline:** These are isotonic solutions, not fixatives. They can keep tissue moist for a very short duration (e.g., during transport for a frozen section), but they do not stop autolysis. Prolonged exposure leads to tissue maceration and cellular degradation. **3. NEET-PG High-Yield Clinical Pearls:** * **Composition:** 10% Formalin is actually a 4% solution of Formaldehyde gas in water. * **Rate of Penetration:** Formalin penetrates tissue at a rate of approximately **1 mm per hour**. * **Glutaraldehyde:** This is the fixative of choice for **Electron Microscopy** as it preserves ultrastructural details better than formalin. * **Carnoy’s Fixative:** A rapid fixative containing alcohol, chloroform, and acetic acid; often used for preserving nucleic acids or identifying glycogen. * **Zenker’s Fluid:** Contains mercuric chloride; historically used for bone marrow biopsies to provide sharp nuclear detail.

Question 376: Which of the following is NOT an integrin ligand?

A. CD 54
B. CD 102
C. CD 106
D. CD 34 (Correct Answer)

Explanation: The process of leukocyte extravasation involves a sequence of molecular interactions: rolling, activation, adhesion, and transmigration. **Integrins** are transmembrane glycoproteins expressed on leukocytes that mediate **firm adhesion** by binding to their specific ligands on the vascular endothelium [1]. **Why CD 34 is the correct answer:** **CD 34** is a sialomucin-like molecule expressed on endothelial cells. It acts as a ligand for **L-selectin** (not integrins) [1]. This interaction is crucial for the initial **rolling** phase of leukocyte recruitment, particularly in high endothelial venules (HEVs) of lymph nodes. Since it binds selectins rather than integrins, it is the correct "NOT" option. **Analysis of incorrect options (Integrin Ligands):** * **CD 54 (ICAM-1):** Intercellular Adhesion Molecule-1 is the primary ligand for the integrins **LFA-1** (CD11a/CD18) and **Mac-1** (CD11b/CD18). It is essential for firm adhesion. * **CD 102 (ICAM-2):** A constituent ligand for integrins, similar to ICAM-1, involved in leukocyte trafficking. * **CD 106 (VCAM-1):** Vascular Cell Adhesion Molecule-1 is the ligand for the integrin **VLA-4** (α4β1). This interaction is vital for the recruitment of lymphocytes, monocytes, and eosinophils. **High-Yield NEET-PG Pearls:** * **Rolling:** Mediated by Selectins (E, P, and L-selectin). Ligands include Sialyl-Lewis X and CD34 [1]. * **Adhesion:** Mediated by Integrins (LFA-1, VLA-4) binding to Immunoglobulin superfamily members (ICAM-1, VCAM-1) [1]. * **Transmigration (Diapedesis):** Mediated primarily by **PECAM-1 (CD31)** [2]. * **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a defect in the **CD18** subunit of integrins, leading to impaired firm adhesion and recurrent bacterial infections without pus formation [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, p. 87. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 87-89.

Question 377: All of the following statements are true for cell aging except?

A. Enlargement of telomere (Correct Answer)
B. Decrease in the number of mitochondria
C. Glycosylation of DNA
D. Glycosylation of RNA

Explanation: **Explanation:** Cellular aging is a progressive decline in cell function and viability caused by genetic abnormalities and the accumulation of cellular and molecular damage. **Why Option A is the Correct Answer (The Exception):** The hallmark of cellular aging is **telomere shortening**, not enlargement [1]. Telomeres are short repeats of DNA sequences (TTAGGG) at the ends of chromosomes that protect them from degradation. With each cell division, a small portion of the telomere is lost (the "end-replication problem"). When telomeres become critically short, the cell enters **replicative senescence**, a state where it stops dividing [1]. Telomerase, the enzyme that maintains telomere length, is absent in most somatic cells but active in germ cells and cancer cells. **Analysis of Incorrect Options:** * **Option B (Decrease in mitochondria):** Aging is associated with a decline in mitochondrial biogenesis and an accumulation of mutations in mitochondrial DNA (mtDNA) [4]. This leads to decreased ATP production and increased generation of Reactive Oxygen Species (ROS) [4]. * **Options C & D (Glycosylation of DNA/RNA):** Aging involves the accumulation of metabolic damage. Non-enzymatic glycosylation (glycation) of macromolecules, including DNA and proteins, leads to the formation of **Advanced Glycation End-products (AGEs)**. These cross-link proteins and damage genetic material, contributing to the aging phenotype. **NEET-PG High-Yield Pearls:** * **Werner Syndrome:** A rare autosomal recessive disorder characterized by premature aging (progeria) caused by a mutation in the *WRN* gene (a DNA helicase), leading to rapid telomere attrition [2]. * **Sirtuins:** A family of NAD+-dependent protein deacetylases (especially SIRT1) that promote longevity by increasing metabolic efficiency and DNA repair [3]. * **Lipofuscin:** Known as the "wear-and-tear" pigment, it is an insoluble brownish-yellow granular intracellular material that accumulates in aging cells (especially heart and liver) [5]. **References:** [1] 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. 243-244. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 77-78. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, p. 79. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 26-27. [5] 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. 241-242.

Question 378: Which type of immune cells are primarily responsible for recognizing and killing cancer cells?

A. Basophils
B. Eosinophils
C. NK cells (Correct Answer)
D. Neutrophils

Explanation: **Explanation:** **1. Why NK cells are the correct answer:** Natural Killer (NK) cells are a type of cytotoxic lymphocyte critical to the **innate immune system** [1]. They play a pivotal role in **immunosurveillance** against tumors [2]. Unlike T-cells, NK cells do not require prior sensitization or MHC-restricted antigen presentation. They identify cancer cells through the **"Missing Self" hypothesis**: many tumor cells downregulate MHC Class I molecules to evade T-cells. NK cells detect this absence of MHC-I and trigger apoptosis in the target cell via the release of perforins and granzymes [1]. **2. Why the other options are incorrect:** * **Basophils (A):** These are granulocytes primarily involved in Type I hypersensitivity reactions (allergy) and defense against ectoparasites. They release histamine and heparin but do not have a primary role in anti-tumor immunity. * **Eosinophils (B):** These are specialized for combating parasitic infections (helminths) and are involved in allergic diseases like asthma. While they can be found in the tumor microenvironment, they are not the primary cells responsible for killing cancer cells. * **Neutrophils (D):** These are the "first responders" of the innate immune system, primarily responsible for phagocytosis and killing of **pyogenic bacteria** and fungi through oxidative burst. **3. High-Yield Clinical Pearls for NEET-PG:** * **Markers:** NK cells are identified by the presence of **CD56** and **CD16**, and the absence of CD3. * **Mechanism:** They are activated by **IL-12, IL-15, and Type I Interferons (IFN-α, IFN-β)** [1]. * **Antibody-Dependent Cellular Cytotoxicity (ADCC):** NK cells can also kill cells coated with IgG antibodies via their CD16 (FcγRIII) receptor. * **Morphology:** On a peripheral smear, they appear as **Large Granular Lymphocytes (LGLs)**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 200-201. [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. 164-165.

Question 379: In apoptosis, Apaf-1 is activated by the release of which of the following substances from the mitochondria?

A. Bcl-2
B. Bax
C. Bcl-XL
D. Cytochrome C (Correct Answer)

Explanation: Apoptosis occurs via two main pathways: the extrinsic (death receptor) pathway and the **intrinsic (mitochondrial) pathway** [1]. The intrinsic pathway is the major mechanism of apoptosis in mammalian cells. **Why Cytochrome C is correct:** When a cell undergoes stress or DNA damage, the permeability of the mitochondrial outer membrane increases. This leads to the leakage of **Cytochrome C** from the intermembrane space into the cytosol [2]. Once in the cytosol, Cytochrome C binds to a cytosolic protein called **Apaf-1** (Apoptotic protease-activating factor-1) [2]. This binding triggers the formation of a wheel-like hexameric complex known as the **Apoptosome**, which subsequently activates Caspase-9, initiating the execution phase of apoptosis [1]. **Why other options are incorrect:** * **Bcl-2 and Bcl-XL (Options A & C):** These are **anti-apoptotic** proteins located in the mitochondrial membrane [1]. They prevent apoptosis by inhibiting the release of Cytochrome C [2]. * **Bax (Option B):** This is a **pro-apoptotic** protein [1]. While Bax promotes the release of Cytochrome C by forming pores in the mitochondrial membrane, it does not directly bind to or activate Apaf-1 [2]. **NEET-PG High-Yield Pearls:** * **The "Master Switch":** The Bcl-2 family regulates the mitochondrial pathway. The ratio of pro-apoptotic (Bax, Bak) to anti-apoptotic (Bcl-2, Bcl-XL) proteins determines cell survival [1]. * **Initiator Caspases:** Caspase-9 is the initiator for the intrinsic pathway; Caspase-8 and 10 are initiators for the extrinsic pathway [1]. * **Executioner Caspases:** Caspase-3 and 6 are common to both pathways [1]. * **Morphology:** Apoptosis is characterized by cell shrinkage and chromatin condensation, but notably **lacks inflammation** (unlike necrosis). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 64-67. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, p.310.

Question 380: Which of the following surface glycoproteins is most often expressed in human hematopoietic stem cells?

A. CD 22
B. CD 40
C. CD 15
D. CD 34 (Correct Answer)

Explanation: **Explanation:** **CD 34** is the hallmark surface marker for **hematopoietic stem cells (HSCs)** and progenitor cells [1]. It is a cell-surface glycoprotein that facilitates cell-to-matrix adhesion, allowing stem cells to anchor within the bone marrow niche. As these cells differentiate into mature lineages, the expression of CD 34 is lost. In clinical practice, CD 34 is used to identify, quantify, and isolate stem cells for peripheral blood stem cell transplantation [2]. **Analysis of Incorrect Options:** * **CD 22:** This is a specific marker for **B-lineage cells**. It is expressed on mature B-cells and is highly useful in diagnosing B-cell lymphomas and leukemias (e.g., Hairy Cell Leukemia). * **CD 40:** This is a costimulatory protein found on **Antigen Presenting Cells (APCs)** like B-cells, macrophages, and dendritic cells. It interacts with CD40L on T-cells, playing a crucial role in B-cell activation and class switching. * **CD 15:** This is a marker for **Granulocytes** and is also classically expressed on Reed-Sternberg cells in **Hodgkin Lymphoma** (along with CD 30). **High-Yield Clinical Pearls for NEET-PG:** * **CD 34+ Count:** Used to determine the adequacy of a stem cell harvest for transplant (minimum threshold is usually $2 \times 10^6$ cells/kg) [2]. * **Acute Leukemia:** CD 34 is often expressed in **blasts** (both AML and ALL), helping to distinguish them from mature lymphoid or myeloid cells [3]. * **Other CD 34+ tumors:** Apart from HSCs, CD 34 is also expressed in vascular tumors (Angiosarcoma) and Solitary Fibrous Tumors. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, pp. 588-589. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 585-586. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 584-585.

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Cell Injury and Cell Death

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Adaptations of Cellular Growth

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Hemodynamic Disorders

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Genetic Disorders

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