Molecular Pathology — MCQs

A 40-year-old man is pulled from the ocean after a boating accident and resuscitated. Six hours later, the patient develops acute renal failure. Kidney biopsy reveals evidence of karyorrhexis and karyolysis in renal tubular epithelial cells. Which of the following biochemical events preceded these pathologic changes?

Molecular Pathology Indian Medical PG Practice Questions and MCQs

Question 41: What is the chromosomal abnormality associated with 100% recurrence of disease in Down's syndrome?

A. Translocation of 15 and 21 chromosomes
B. Mosaic pattern
C. Trisomy 21 with translocation (Correct Answer)
D. Non-disjunction

Explanation: **Explanation:** The question focuses on the genetic risk of recurrence in Down Syndrome (Trisomy 21). While most cases are sporadic, specific chromosomal arrangements carry a significantly higher risk of recurrence in future offspring. **1. Why Option C is Correct:** The "100% recurrence" specifically refers to a **Robertsonian Translocation** involving chromosome 21. If a parent is a carrier of a **21q;21q translocation** (where two 21st chromosomes are fused together), they can only produce gametes that either have the fused 21;21 chromosome or no 21st chromosome at all [1]. Consequently, all viable offspring will inherit the fused pair plus a normal 21 from the other parent, resulting in **obligate Trisomy 21**. **2. Why Incorrect Options are Wrong:** * **Option A (Translocation 15;21):** This is the most common translocation in Down Syndrome (approx. 4%) [1]. However, the recurrence risk for a carrier mother is only about 10-15%, and for a father, it is 1-3%. It does not lead to 100% recurrence. * **Option B (Mosaicism):** This occurs due to mitotic non-disjunction during early embryogenesis [1]. The risk of recurrence is generally the same as the general population (very low). * **Option D (Non-disjunction):** This is the cause of **95% of Down Syndrome cases** (Meiotic non-disjunction, mostly maternal) [1]. It is a sporadic event related to advanced maternal age, with a low recurrence risk (approx. 1%). **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause:** Meiotic non-disjunction (95%), specifically in Meiosis I of the oocyte. * **Robertsonian Translocation:** Accounts for 4% of cases; independent of maternal age [1]. * **21q;21q Translocation:** The only scenario with a **100% theoretical risk** of Down Syndrome in viable offspring. * **Screening:** Low AFP, Low Estriol, High hCG, and High Inhibin-A (The "Quad Test" profile). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 170-171.

Question 42: Which of the following is true about metastatic calcification?

A. Serum calcium level is normal
B. Occurs in dead or dying tissue
C. Occurs in damaged heart valves
D. Calcification starts in mitochondria (Correct Answer)

Explanation: **Explanation:** Pathologic calcification is the abnormal tissue deposition of calcium salts. It is divided into two types: **Dystrophic** and **Metastatic**. **Why Option D is Correct:** In **metastatic calcification**, the process typically begins in the **mitochondria** of cells that secrete acid (like gastric mucosa, kidneys, and lungs). The mitochondria act as the initial site of mineral concentration. In contrast, dystrophic calcification usually initiates in membrane-bound vesicles (derived from degenerating cells). **Analysis of Incorrect Options:** * **Option A:** In metastatic calcification, the **serum calcium level is elevated** (hypercalcemia) [2]. Normal serum calcium levels are characteristic of dystrophic calcification. * **Option B:** Metastatic calcification occurs in **normal, living tissues** due to systemic hypercalcemia [2]. Calcification in dead or dying tissue is the definition of dystrophic calcification. * **Option C:** Damaged heart valves (e.g., chronic rheumatic heart disease) undergo **dystrophic calcification** because the tissue is injured, even though serum calcium levels remain normal. **NEET-PG High-Yield Pearls:** 1. **Favored Sites:** Metastatic calcification occurs in tissues with an **internal alkaline compartment** due to the excretion of acid: Gastric mucosa, Kidneys, Lungs, Systemic arteries, and Pulmonary veins [1]. 2. **Common Causes:** Hyperparathyroidism (most common), Vitamin D intoxication, Bone resorption (multiple myeloma, bony metastasis), and Renal failure (secondary hyperparathyroidism) [1], [2]. 3. **Morphology:** On H&E stain, both types appear as basophilic, amorphous granular clumps [1]. Von Kossa stain (black) and Alizarin Red S (red) are used to confirm calcium. **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. 76-77. [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. 127-135.

Question 43: BRCA1 and BRCA2 genes are located on which chromosomes?

A. Chromosome 13 and 17 (Correct Answer)
B. Chromosome 17 and 13
C. Chromosome 11 and 13
D. Chromosome 13 and 11

Explanation: The correct answer is **A (Chromosome 13 and 17)**. This question tests the specific chromosomal localization of tumor suppressor genes involved in DNA repair [2]. * **BRCA1** is located on the long arm of **Chromosome 17** (specifically 17q21). * **BRCA2** is located on the long arm of **Chromosome 13** (specifically 13q12.3). The sequence in the question (13 and 17) corresponds to the order of the genes mentioned (BRCA1 and BRCA2). Both genes encode proteins essential for **homologous recombination repair** of double-stranded DNA breaks [2]. Mutations in these genes significantly increase the risk of hereditary breast and ovarian cancer (HBOC) syndrome [3]. **Analysis of Incorrect Options:** * **Option B (17 and 13):** While these are the correct chromosomes, the order is reversed. In medical entrance exams like NEET-PG, the sequence of numbers must match the sequence of the entities named in the stem. * **Options C & D (11 and 13):** Chromosome 11 is associated with other significant pathologies (e.g., WT1 gene for Wilms tumor, β-globin chain synthesis), but not BRCA1. **High-Yield Clinical Pearls for NEET-PG:** * **Function:** BRCA1/2 are "Caretaker" tumor suppressor genes [2]. * **Cancer Risks:** BRCA1 carries a higher risk for ovarian cancer compared to BRCA2. BRCA2 is more strongly associated with **male breast cancer** and pancreatic cancer [1], [3]. * **Inheritance:** Autosomal Dominant with variable penetrance. * **Treatment:** Tumors with BRCA mutations are highly sensitive to **PARP inhibitors** (e.g., Olaparib) due to the principle of "synthetic lethality." * **Mnemonic:** BRCA**1** is on **1**7; BRCA**2** is on **1**3. (Note that both are on the long arm 'q'). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Pancreas, pp. 898-899. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, p. 300. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Breast, pp. 1058-1059.

Question 44: What change is typically observed in mitochondria due to aging?

A. Decrease in size and increase in number
B. Decrease in number and increase in size
C. Decrease in both size and number (Correct Answer)
D. Increase in both size and number

Explanation: **Explanation:** The correct answer is **C: Decrease in both size and number.** **Underlying Medical Concept:** Aging is characterized by a progressive decline in cellular function and metabolic efficiency. In the context of mitochondria, this is primarily driven by the **Free Radical Theory of Aging**. Over time, mitochondria accumulate mutations in their mitochondrial DNA (mtDNA) due to constant exposure to Reactive Oxygen Species (ROS) generated during oxidative phosphorylation [1], [2]. As cells age: 1. **Biogenesis decreases:** The production of new mitochondria slows down. 2. **Mitophagy increases/dysfunctions:** Damaged mitochondria are cleared out, but the replacement rate is insufficient [3]. 3. **Morphological changes:** There is a characteristic **reduction in both the total number of mitochondria and their individual size**, leading to a diminished capacity for ATP production and increased cellular senescence. **Analysis of Incorrect Options:** * **A & B:** These options suggest a compensatory mechanism (increase in size or number). While some cells may initially undergo "mitochondrial swelling" or compensatory hypertrophy in response to acute stress, the hallmark of *aging* is a net loss of organelle mass and density [1]. * **D:** An increase in both size and number is typically seen in high-metabolic states or physiological hypertrophy (e.g., skeletal muscle following exercise training), which is the opposite of the aging process. **High-Yield NEET-PG Pearls:** * **Mitochondrial DNA (mtDNA):** It is more susceptible to damage than nuclear DNA because it lacks protective histones and has limited repair mechanisms [2]. * **Morphological Hallmarks of Aging:** Apart from mitochondrial shrinkage, look for accumulation of **Lipofuscin** (the "wear-and-tear" pigment) and telomere shortening [3]. * **Werner Syndrome:** A high-yield progeroid syndrome (premature aging) caused by a mutation in the *WRN* gene (DNA helicase). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 26-27. [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. 100-101. [3] 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 45: Which of the following vitamins is required for post-translational modification?

A. Vitamin B12
B. Biotin
C. Beta-carotene
D. Vitamin C (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Vitamin C** Vitamin C (Ascorbic acid) is a critical cofactor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the **hydroxylation** of proline and lysine residues in pre-procollagen chains. This hydroxylation is a classic example of **post-translational modification**, occurring after the polypeptide chain has been synthesized. It allows for the formation of stable hydrogen bonds, which are essential for the triple-helix stability of collagen. Without Vitamin C, collagen fibers are defective and weak, leading to the clinical manifestation of Scurvy. **Analysis of Incorrect Options:** * **Vitamin B12 (Cobalamin):** Acts as a coenzyme for DNA synthesis (conversion of homocysteine to methionine) and myelin maintenance. It is involved in metabolic pathways rather than post-translational protein modification. * **Biotin (Vitamin B7):** Functions as a cofactor for **carboxylation** enzymes (e.g., Pyruvate carboxylase). While it modifies substrates, it is not primarily associated with the structural modification of proteins after translation. * **Beta-carotene:** A precursor to Vitamin A. Vitamin A is involved in gene transcription (via RAR/RXR receptors) and epithelial differentiation, but not direct post-translational modification of proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Characterized by "corkscrew hair," perifollicular hemorrhages, and bleeding gums due to defective collagen. * **Vitamin K:** Another high-yield vitamin involved in post-translational modification (**gamma-carboxylation** of Glutamic acid residues on Factors II, VII, IX, and X) [1]. * **Location:** Hydroxylation of collagen occurs within the **Rough Endoplasmic Reticulum (RER)**. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 582-583, 624-625.

Question 46: In Langerhans Cell Histiocytosis, what is the characteristic abnormality seen?

A. Foamy macrophages
B. Giant cells
C. Plasma cells
D. Birbeck's granules (Correct Answer)

Explanation: **Explanation:** **Langerhans Cell Histiocytosis (LCH)** is a clonal proliferation of Langerhans cells, which are specialized dendritic cells [2]. The hallmark diagnostic feature of LCH is the presence of **Birbeck’s granules** (Option D) [1]. * **Why Birbeck’s granules are correct:** These are unique, rod-shaped, pentalaminar cytoplasmic organelles with a dilated terminal end, giving them a characteristic **"tennis racket" appearance** on Electron Microscopy (EM) [1]. They contain the protein **Langerin (CD207)**, which is involved in the endocytosis of antigens [1]. In LCH, cells also typically express **CD1a** and **S100**. **Analysis of Incorrect Options:** * **A. Foamy macrophages:** These are lipid-laden macrophages commonly seen in atherosclerosis, xanthomas, or Niemann-Pick disease, but are not specific to LCH. * **B. Giant cells:** While multinucleated giant cells can occasionally be seen in the background of LCH lesions, they are non-specific and found in various granulomatous inflammations and bone tumors. * **C. Plasma cells:** These are seen in chronic inflammation and Multiple Myeloma. While LCH lesions have an "inflammatory" background (eosinophils, lymphocytes), plasma cells are not the defining feature. **High-Yield Clinical Pearls for NEET-PG:** * **BRAF V600E Mutation:** Seen in approximately 50-60% of LCH cases (crucial molecular marker) [2]. * **Eosinophilic Granuloma:** The most common clinical presentation of LCH, often appearing as a "punched-out" bone lesion (especially in the skull). * **Hand-Schüller-Christian triad:** Diabetes insipidus, Exophthalmos, and Bone lesions. * **Letterer-Siwe disease:** The aggressive, multisystem form seen in infants. **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. 630. [2] 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. 629-630.

Question 47: Liquefactive necrosis is typically seen in which of the following conditions?

A. Ischemic necrosis of the heart
B. Ischemic necrosis of the brain (Correct Answer)
C. Ischemic necrosis of the intestine
D. Tuberculosis

Explanation: **Explanation:** **Liquefactive necrosis** is characterized by the transformation of the tissue into a liquid, viscous mass. This occurs because the rate of enzymatic digestion of cells exceeds the rate of protein denaturation. **Why Option B is Correct:** In the **Central Nervous System (CNS)**, ischemic injury (stroke) uniquely results in liquefactive necrosis rather than coagulative necrosis [1]. This is due to two primary reasons: 1. **High Lipid Content:** The brain is rich in lipids and low in supporting connective tissue. 2. **Hydrolytic Enzymes:** Brain cells contain a high concentration of lysosomal enzymes that rapidly digest the dead tissue, leading to the formation of a soft, liquefied area that eventually results in a cystic space [1]. **Analysis of Incorrect Options:** * **Option A (Heart):** Ischemic necrosis of the heart (Myocardial Infarction) leads to **Coagulative Necrosis**. The cell's structural proteins are denatured, preserving the basic outline of the cell for several days. * **Option C (Intestine):** Ischemic necrosis of the bowel typically results in **Gangrenous Necrosis**. While this starts as coagulative necrosis, if a bacterial infection superimposes, it becomes "wet gangrene" (a form of liquefactive necrosis), but the primary ischemic process is coagulative. * **Option D (Tuberculosis):** This is the classic example of **Caseous Necrosis**, characterized by a "cheese-like," friable, white appearance, typically found within a granuloma. **NEET-PG High-Yield Pearls:** * **Coagulative Necrosis:** The most common pattern; seen in all solid organ infarcts **except** the brain. * **Liquefactive Necrosis:** Seen in two specific scenarios: **Brain infarcts** and **Abscesses** (due to neutrophilic enzymes) [1]. * **Fat Necrosis:** Seen in Acute Pancreatitis (enzymatic) and breast trauma (non-enzymatic). * **Fibrinoid Necrosis:** Seen in immune-mediated vascular damage (e.g., Polyarteritis Nodosa, Malignant Hypertension). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Central Nervous System, pp. 1268-1269.

Question 48: Which of the following stem cells are lowest in the hierarchy of specialization?

A. Unipotent stem cells
B. Totipotent stem cells (Correct Answer)
C. Multipotent uncommitted stem cells
D. Committed stem cells

Explanation: ### Explanation The hierarchy of stem cells is defined by their **potency**, which refers to the cell's ability to differentiate into different cell types. In biological hierarchy, the "lowest" level of specialization (or the "highest" level of potency) belongs to cells that have not yet committed to any specific lineage. **Why Totipotent Stem Cells are Correct:** **Totipotent stem cells** (e.g., the zygote and early blastomeres) are at the absolute top of the potency hierarchy but the **lowest in specialization** [1]. They have the "total" potential to differentiate into any cell type in the body, including **extra-embryonic tissues** (like the placenta) [1]. Because they are completely undifferentiated and uncommitted, they represent the starting point of development [2]. **Analysis of Incorrect Options:** * **Multipotent uncommitted stem cells:** These are further down the hierarchy. They can differentiate into multiple cell types but are restricted to a specific family (e.g., Hematopoietic stem cells can form all blood cells but not neurons) [1]. * **Committed stem cells:** These are progenitor cells that have already "decided" their fate [2]. They are more specialized and have lost the ability to produce diverse cell types [3]. * **Unipotent stem cells:** These are at the **highest level of specialization** and the lowest level of potency [1]. They can only produce one specific cell type (e.g., basal cells of the epidermis) but retain the property of self-renewal. **NEET-PG High-Yield Pearls:** 1. **Hierarchy Order (Potency):** Totipotent > Pluripotent (can form all three germ layers but NOT placenta) [1] > Multipotent > Oligopotent > Unipotent. 2. **Pluripotent Example:** Embryonic Stem Cells (ESCs) derived from the inner cell mass of the blastocyst [2]. 3. **Induced Pluripotent Stem Cells (iPS):** Somatic cells "reprogrammed" to a pluripotent state using specific transcription factors (Yamanaka factors: Oct3/4, Sox2, Klf4, and c-Myc). 4. **Niche:** The specific microenvironment that maintains stem cell dormancy and regulates their transition to proliferation. **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. 84-85. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 39-40. [3] 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. 77-78.

Question 49: Which of the following is NOT involved in phagocytosis?

A. Recognition and attachment
B. Engulfment
C. Pavementing of cells (Correct Answer)
D. Killing and degradation

Explanation: ### Explanation **Phagocytosis** is a specific step in the cellular events of acute inflammation, defined as the process of ingestion of particulate matter (such as bacteria or cell debris) by neutrophils and macrophages. **1. Why "Pavementing of cells" is the correct answer:** Pavementing (also known as **Margination**) occurs during the **hemodynamic phase** of inflammation [4]. As blood flow slows (stasis), leukocytes move from the central axial stream to the periphery of the vessel wall [4]. While it is a prerequisite for leukocytes to eventually reach the site of injury, it is a **vascular event**, not a component of the phagocytic process itself. **2. Analysis of Incorrect Options (Steps of Phagocytosis):** Phagocytosis consists of three distinct, sequential steps [1]: * **Recognition and Attachment (Option A):** Phagocytes use receptors (like Mannose or Scavenger receptors) to bind to microbes [1]. This is enhanced by **opsonins** (IgG, C3b) [3]. * **Engulfment (Option B):** The cell membrane extends pseudopods around the particle to form a **phagosome**, which then fuses with a lysosome to form a **phagolysosome** [1], [2]. * **Killing and Degradation (Option D):** This occurs via **Oxygen-dependent** mechanisms (Respiratory burst involving NADPH oxidase and Myeloperoxidase) or **Oxygen-independent** mechanisms (Lysozyme, Lactoferrin, Acid hydrolases) [2]. **Clinical Pearls for NEET-PG:** * **Opsonization:** The most important opsonins are **IgG** and **C3b** [1], [3]. * **NADPH Oxidase Deficiency:** Leads to **Chronic Granulomatous Disease (CGD)**, where phagocytes can engulf but cannot kill catalase-positive organisms. * **Chédiak-Higashi Syndrome:** A defect in vesicle fusion (phagolysosome formation), leading to impaired engulfment and giant granules in neutrophils. * **Sequence of Cellular Events:** Margination → Rolling → Adhesion → Diapedesis (Transmigration) → Chemotaxis → **Phagocytosis** [4]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 89-91. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, p. 91. [3] 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. 190-191. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Migration in the tissues toward a chemotactic stimulus, pp. 86-87.

Question 50: A 40-year-old man is pulled from the ocean after a boating accident and resuscitated. Six hours later, the patient develops acute renal failure. Kidney biopsy reveals evidence of karyorrhexis and karyolysis in renal tubular epithelial cells. Which of the following biochemical events preceded these pathologic changes?

A. Activation of Na+/K+ ATPase
B. Decrease in intracellular calcium
C. Decrease in intracellular pH (Correct Answer)
D. Increase in ATP production

Explanation: **Explanation:** The clinical scenario describes **ischemic acute tubular necrosis (ATN)** resulting from hypoperfusion (near-drowning/shock). Karyorrhexis and karyolysis are definitive markers of **irreversible cell injury** (necrosis) [1]. **Why "Decrease in intracellular pH" is correct:** When blood flow to the kidneys decreases, hypoxia ensues, leading to a failure of oxidative phosphorylation [1]. The cell shifts to **anaerobic glycolysis** to maintain ATP levels. This metabolic shift results in the accumulation of **lactic acid** and inorganic phosphates. The consequent drop in intracellular pH (acidosis) causes **clumping of nuclear chromatin**, which is one of the earliest light microscopic changes preceding nuclear dissolution (karyolysis) [1]. **Analysis of Incorrect Options:** * **A & D:** In ischemia, there is a **decrease in ATP production**, not an increase [1]. This leads to the **failure (not activation) of the Na+/K+ ATPase pump**, causing intracellular sodium accumulation and cellular swelling (hydropic change) [1]. * **B:** Ischemia leads to an **increase in intracellular calcium**. Failure of ATP-dependent calcium pumps causes calcium to influx from the extracellular space and leak from the mitochondria/ER [1]. High cytosolic calcium activates injurious enzymes like phospholipases, proteases, and endonucleases [1]. **NEET-PG High-Yield Pearls:** * **Earliest change in reversible injury:** Cellular swelling (due to Na+/K+ ATPase failure) [1]. * **Point of no return (Irreversible injury):** Severe mitochondrial damage (amorphous densities) and profound membrane damage [1]. * **Nuclear changes in necrosis:** Pyknosis (shrinkage/condensation) → Karyorrhexis (fragmentation) → Karyolysis (dissolution by DNase) [1]. * **Ischemic ATN:** Characterized by "skip lesions" along the nephron, most severely affecting the proximal convoluted tubule (PCT) and thick ascending limb. **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. 51-61.

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