General Pathology — MCQs

General Pathology Indian Medical PG Practice Questions and MCQs

Question 651: What is true about MHC?

A. Present on chromosome
B. Class II comprises A, B, C loci
C. Class III has complement (Correct Answer)
D. Class I is involved in mixed leucocyte reaction

Explanation: The Major Histocompatibility Complex (MHC), known as **HLA (Human Leukocyte Antigen)** in humans, is a cluster of genes located on the **short arm of Chromosome 6** [1]. It plays a critical role in immune recognition and antigen presentation. ### **Explanation of Options** * **Correct Answer (C):** The MHC locus is divided into three classes. **Class III genes** do not encode antigen-presenting molecules; instead, they encode various components of the innate immune system, most notably **complement proteins (C2, C4, and Factor B)**, as well as cytokines like TNF-α and TNF-β. * **Option A (Incorrect):** While MHC is present on a chromosome, the statement is incomplete/vague [1]. In competitive exams, specific functional truths (like Class III contents) take precedence over general structural facts. * **Option B (Incorrect):** **Class I** comprises the A, B, and C loci. **Class II** comprises the **DP, DQ, and DR** loci [1]. * **Option C (Incorrect):** The Mixed Leukocyte Reaction (MLR) is primarily a test for **MHC Class II** compatibility (specifically the HLA-DR locus), as Class II molecules trigger the proliferation of T-helper cells [3,4]. ### **High-Yield Clinical Pearls for NEET-PG** * **Class I MHC:** Present on all nucleated cells and platelets (absent on RBCs) [1]. It presents endogenous antigens to **CD8+ T-cells** [4]. * **Class II MHC:** Present only on **Antigen Presenting Cells (APCs)** like dendritic cells, macrophages, and B-cells [1,2]. It presents exogenous antigens to **CD4+ T-cells** [2,3]. * **Structure:** Class I consists of one heavy chain and a **β2-microglobulin** (encoded on Chromosome 15). Class II consists of two polypeptide chains (α and β), both encoded within the MHC locus [1,2]. * **HLA Association:** HLA-B27 is strongly associated with Ankylosing Spondylitis; HLA-DR3/DR4 with Type 1 Diabetes Mellitus [3]. **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. 156-157. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 202-203. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 203-204. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 240-241.

Question 652: Wear and tear pigment in the body refers to which of the following?

A. Lipochrome (Correct Answer)
B. Melanin
C. Anthracotic pigment
D. Haemosiderin

Explanation: **Explanation:** **Lipochrome** (also known as **Lipofuscin**) is the correct answer. It is famously referred to as the "wear-and-tear" or "aging" pigment [1]. It is an insoluble, brownish-yellow granular intracellular pigment that accumulates in cells as they age or undergo atrophy. * **Mechanism:** It is a product of **lipid peroxidation** of polyunsaturated lipids of subcellular membranes. It represents "indigestible" material stored within lysosomes (residual bodies) following autophagy. It is most commonly seen in permanent cells that do not divide, such as **cardiac myocytes** [1] and **neurons**, as well as the liver. **Why other options are incorrect:** * **Melanin:** This is an endogenous, brown-black pigment produced by melanocytes in the basal layer of the epidermis [1]. Its primary function is protection against UV radiation, not a marker of cellular aging. * **Anthracotic pigment:** This is an **exogenous** pigment (carbon/coal dust). It is inhaled from the atmosphere and phagocytosed by alveolar macrophages, commonly seen in the lungs and hilar lymph nodes. * **Haemosiderin:** This is a golden-yellow to brown hemoglobin-derived pigment that represents large aggregates of ferritin. It serves as a form of iron storage and accumulates in areas of hemorrhage or systemic iron overload (hemosiderosis). **High-Yield NEET-PG Pearls:** 1. **Brown Atrophy:** When heavy deposits of lipofuscin are accompanied by organ shrinkage (atrophy), the condition is termed "Brown Atrophy" (classically seen in the heart). 2. **Staining:** Lipofuscin is **PAS positive** and can be visualized with Sudan Black B. 3. **Significance:** It is not toxic to the cell itself but serves as a hallmark of free radical injury and lipid peroxidation. **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, p. 75.

Question 653: Caseation necrosis is suggestive of which of the following conditions?

A. Tuberculosis (Correct Answer)
B. Sarcoidosis
C. Leprosy
D. Midline lethal granuloma

Explanation: **Explanation:** **Caseation necrosis** is a unique form of cell death that combines features of both coagulative and liquefactive necrosis [1]. It is histologically characterized by a complete loss of cellular architecture, appearing as "cheese-like" (caseous), friable, white-yellow debris [1]. **Why Tuberculosis is the correct answer:** The hallmark of *Mycobacterium tuberculosis* infection is the formation of a **tuberculous granuloma** [2]. The high lipid content (mycolic acids) in the cell wall of the mycobacteria, combined with the host's delayed-type hypersensitivity (Type IV) response, leads to the formation of this central necrotic area. Unlike other granulomatous diseases, TB is the classic prototype for **caseating granulomas** [2]. **Analysis of Incorrect Options:** * **B. Sarcoidosis:** Characterized by **non-caseating granulomas**. The absence of central necrosis is a key diagnostic feature used to differentiate it from TB. * **C. Leprosy:** Tuberculoid leprosy involves granuloma formation, but these are typically **non-caseating**. While rare instances of "nerve abscesses" occur, caseation is not the defining feature. * **D. Midline Lethal Granuloma:** Now largely classified under NK/T-cell lymphomas, this condition presents with extensive **coagulative necrosis** due to vascular destruction (angiocentricity), rather than caseation. **NEET-PG High-Yield Pearls:** * **Microscopic appearance:** Caseous necrosis appears as structureless, eosinophilic (pink), granular debris surrounded by a rim of epithelioid histiocytes, Langhans giant cells, and lymphocytes [2]. * **Dystrophic Calcification:** Caseous centers often undergo calcification (e.g., Ghon complex in TB). * **Mnemonic:** "Caseous" = "Cheese-like." If you see "non-caseating" on the exam, think Sarcoidosis or Crohn's disease first. **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, p. 55. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Infectious Diseases, pp. 383-384.

Question 654: Fibrinoid necrosis can be seen in:

A. Abscess cavity.
B. Pancreas.
C. Heart.
D. Peptic ulcer. (Correct Answer)

Explanation: **Explanation:** **Fibrinoid necrosis** is a specialized form of cell death characterized by the deposition of immune complexes and plasma proteins (like fibrin) into the walls of blood vessels or damaged tissues [3]. On H&E staining, it appears as a bright pink, amorphous, "fibrin-like" material. **Why Peptic Ulcer is Correct:** In a **Peptic Ulcer**, the base of the ulcer undergoes intense inflammation and digestion [2]. The floor of the ulcer typically consists of four layers (from superficial to deep): necrotic debris, non-specific inflammation, **granulation tissue with fibrinoid necrosis**, and finally, cicatrization (fibrosis). The fibrinoid change occurs due to the leakage of plasma proteins into the damaged vessel walls at the ulcer base. **Analysis of Incorrect Options:** * **A. Abscess cavity:** This is the classic site for **Liquefactive necrosis**, where pyogenic bacteria trigger an accumulation of inflammatory cells (neutrophils) that release enzymes, turning the tissue into a liquid viscous mass (pus). * **B. Pancreas:** Acute pancreatitis is the hallmark site for **Enzymatic Fat necrosis**. Activated lipases release fatty acids from triglycerides, which then combine with calcium to form chalky white deposits (saponification). * **C. Heart:** Myocardial infarction leads to **Coagulative necrosis**, where the cell architecture is preserved for several days despite cell death (ghost cells). *Note: Fibrinoid necrosis can occur in the heart specifically in Rheumatic Heart Disease (Aschoff bodies), but it is not the primary necrotic process for the organ as a whole.* **High-Yield Pearls for NEET-PG:** 1. **Classic Sites for Fibrinoid Necrosis:** Malignant hypertension (arterioles) [3], Polyarteritis Nodosa (PAN) [1], Immune complex-mediated vasculitis, and the base of Peptic Ulcers. 2. **Aschoff Bodies:** In Rheumatic Fever, the central focus of the granuloma contains fibrinoid necrosis. 3. **Appearance:** It is the only necrosis that is primarily identified by its **microscopic** appearance rather than gross morphology. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 518-519. [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. 107-108. [3] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Cardiovascular Disease, pp. 277-278.

Question 655: Thickening of ventricular walls is due to what cellular process?

A. Metaplasia
B. Anaplasia
C. Hypertrophy (Correct Answer)
D. Hyperplasia

Explanation: **Explanation:** The correct answer is **Hypertrophy**. **1. Why Hypertrophy is correct:** Hypertrophy is defined as an increase in the **size of cells**, resulting in an increase in the size of the organ [1], [3]. This process occurs in cells that have a limited capacity to divide (permanent cells), such as cardiac myocytes and skeletal muscle [3]. When the heart faces an increased workload (e.g., systemic hypertension or aortic stenosis), the cardiac myocytes synthesize more proteins and organelles to handle the stress, leading to the thickening of the ventricular walls [1], [2]. **2. Why the other options are incorrect:** * **Hyperplasia:** This is an increase in the **number of cells** [3]. Since adult cardiac myocytes are permanent cells and cannot undergo mitosis, they cannot undergo hyperplasia to any significant degree [3]. * **Metaplasia:** This is a reversible change where one adult cell type is replaced by another (e.g., Squamous metaplasia in a smoker's airway). It does not result in wall thickening. * **Anaplasia:** This refers to a lack of differentiation and is a hallmark of malignancy. It is not a physiological or adaptive response to workload. **Clinical Pearls for NEET-PG:** * **Mechanism:** Hypertrophy is mediated by the induction of genes (like *c-fos, c-jun*), growth factors (IGF-1), and vasoactive agents (Endothelin-1, Angiotensin II) [1]. * **Pure Hypertrophy:** Occurs in the **Heart** and **Skeletal muscle** [1]. * **Combined Hypertrophy & Hyperplasia:** Occurs in the **Uterus during pregnancy** [1]. * **Pathological vs. Physiological:** Ventricular thickening due to hypertension is pathological, whereas the "Athlete’s heart" is a physiological adaptation [2]. **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. 45-46. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Heart, p. 536. [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. 85-87.

Question 656: What is the most common trisomy among the following?

A. Trisomy 18
B. Trisomy 21 (Correct Answer)
C. Trisomy 13
D. Trisomy 5

Explanation: **Explanation:** **Trisomy 21 (Down Syndrome)** is the most common chromosomal disorder [1] and the most frequent cause of intellectual disability of genetic origin [3]. It occurs in approximately 1 in 700 live births [3]. The primary mechanism is **meiotic non-disjunction**, which is strongly associated with advanced maternal age [1]. It is the most common trisomy among live-born infants because the genetic imbalance is relatively well-tolerated compared to other autosomes, allowing for higher survival rates in utero and postnatally [4]. **Analysis of Incorrect Options:** * **Trisomy 18 (Edwards Syndrome):** This is the second most common autosomal trisomy [2]. It is characterized by severe malformations (e.g., rocker-bottom feet, clenched fists with overlapping fingers) and has a very high mortality rate within the first year of life [3]. * **Trisomy 13 (Patau Syndrome):** The third most common live-born trisomy [2]. It presents with midline defects like holoprosencephaly, cleft lip/palate, and polydactyly [3]. Survival beyond infancy is rare. * **Trisomy 5:** Autosomal trisomies involving larger chromosomes (like chromosome 5) are generally incompatible with life and typically result in early spontaneous abortion [2]. (Note: *Deletion* of the short arm of chromosome 5 causes Cri-du-chat syndrome). **High-Yield Clinical Pearls for NEET-PG:** * **Most common cause of Trisomy 21:** Maternal meiotic non-disjunction (95% of cases). * **Robertsonian Translocation:** Accounts for ~4% of cases; unlike non-disjunction, this carries a high risk of recurrence in future pregnancies [5]. * **Cardiac Association:** Endocardial cushion defects (Atrioventricular Septal Defects) are the most common congenital heart lesions in Down Syndrome. * **Hematologic Risk:** Increased risk of **ALL** (Acute Lymphoblastic Leukemia) and **AML M7** (Acute Megakaryoblastic Leukemia). * **Neuropathology:** Virtually all patients develop Alzheimer’s-like neurofibrillary tangles by age 40. **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] 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. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 171-172. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 168-169. [5] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 170-171.

Question 657: Which one of the following statements is FALSE regarding autosomal dominant disorders?

A. Mutated genes can express themselves in the heterozygous state.
B. The recurrence risk is 25% for parents with a previously affected child. (Correct Answer)
C. Males and females are equally affected.
D. These disorders typically involve defects of structural proteins or receptors.

Explanation: In Autosomal Dominant (AD) disorders, a single copy of the mutated gene (heterozygous state) is sufficient to cause the disease [1]. **Explanation of the Correct Answer (Option B):** The statement is **FALSE** because the recurrence risk for an AD disorder is **50%**, not 25% [1]. In a typical scenario where one parent is affected (Aa) and the other is unaffected (aa), each child has a 1 in 2 chance of inheriting the mutant allele. A 25% recurrence risk is characteristic of Autosomal Recessive (AR) inheritance, where both parents are asymptomatic carriers [1]. **Analysis of Incorrect Options:** * **Option A:** This is **True**. AD disorders manifest in the heterozygous state (Aa). * **Option C:** This is **True**. Because the gene is located on an autosome (non-sex chromosome), the disorder affects males and females with equal frequency and severity. * **Option D:** This is **True**. AD disorders typically involve **structural proteins** (e.g., Collagen in Osteogenesis Imperfecta, Fibrillin in Marfan Syndrome) or **receptors/regulatory proteins** [2]. In contrast, AR disorders usually involve enzyme deficiencies [2]. **High-Yield NEET-PG Pearls:** 1. **Reduced Penetrance:** Some individuals inherit the mutant gene but do not express the phenotype (e.g., Retinoblastoma). 2. **Variable Expressivity:** Individuals with the same genotype show different degrees of clinical severity (e.g., Neurofibromatosis Type 1). 3. **Delayed Onset:** Symptoms may not appear until adulthood (e.g., Huntington’s Disease, Adult Polycystic Kidney Disease) [3]. 4. **De Novo Mutations:** A child may be affected even if both parents are normal due to a new mutation in the germ cell (often associated with advanced paternal age). **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. 53-54. [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. 57-58. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 149-150.

Question 658: Caseous necrosis is not found in which of the following conditions?

A. Tuberculosis
B. Histoplasmosis
C. Cytomegalovirus infection (Correct Answer)
D. Syphilis

Explanation: **Explanation:** **Caseous necrosis** is a unique form of cell death that combines features of both coagulative and liquefactive necrosis [1]. Macroscopically, it appears "cheese-like" (friable and white), and microscopically, it is characterized by a structureless, eosinophilic, granular debris surrounded by a granulomatous inflammatory border [2]. **Why Cytomegalovirus (CMV) is the correct answer:** CMV infection typically results in **liquefactive necrosis** (especially in the brain) or focal necrosis without the characteristic "cheesy" granulomatous appearance. The hallmark of CMV is the presence of **"Owl’s eye" intranuclear inclusion bodies**, not caseation. **Analysis of incorrect options:** * **Tuberculosis (TB):** This is the classic prototype of caseous necrosis [1]. It is caused by the body's delayed-type hypersensitivity response to *Mycobacterium tuberculosis*. * **Histoplasmosis:** Fungal infections, particularly *Histoplasma capsulatum* and *Coccidioides*, frequently mimic TB by forming necrotizing (caseating) granulomas. * **Syphilis:** While the classic lesion of tertiary syphilis is the **Gumma** (which shows "gummatous necrosis"), it is considered a variant of caseous necrosis where the tissue architecture is slightly more preserved (rubbery) compared to TB [4]. **NEET-PG High-Yield Pearls:** 1. **Caseous Necrosis:** Architecture is completely obliterated (unlike coagulative necrosis) [1]. 2. **Ghon Complex:** The combination of a parenchymal lung lesion and nodal involvement in primary TB, both showing caseation. 3. **Non-caseating Granulomas:** Think of Sarcoidosis, Crohn’s disease, and Berylliosis [3]. 4. **CMV Hallmark:** Large cells (cytomegaly) with prominent basophilic intranuclear inclusions surrounded by a clear halo. **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, p. 55. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Manifestations Of Central And Peripheral Nervous System Disease, pp. 741-742. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, p. 109. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Infectious Diseases, pp. 360-362.

Question 659: Appearance of spleen with deposits of Amyloid within the white pulp of spleen is known as?

A. Sago spleen (Correct Answer)
B. Lardaceous spleen
C. Nutmeg spleen
D. Zahn spleen

Explanation: In amyloidosis, the spleen is one of the most commonly affected organs. The pattern of deposition depends on the specific anatomical site involved: **1. Why "Sago Spleen" is correct:** When amyloid deposits are limited primarily to the **splenic follicles (white pulp)**, they appear as macroscopic, pale, translucent grains against the red background of the normal splenic pulp. This resembles grains of **Sago** (a starch derived from palm stems). Microscopically, the amyloid replaces the lymphoid follicles. **2. Why the other options are incorrect:** * **Lardaceous Spleen:** This occurs when amyloid deposition involves the **red pulp** (splenic sinuses and cords) rather than the white pulp. The deposits coalesce into large, map-like areas, giving the organ a firm, waxy appearance resembling "lard" (pig fat). * **Nutmeg Spleen:** This refers to the speckled appearance of the liver (not spleen) seen in **Chronic Passive Congestion (CPC)**, usually due to right-sided heart failure. * **Zahn Spleen (Infarcts of Zahn):** These are pseudo-infarcts of the **liver** caused by thrombosis of the portal vein branches; they are not related to amyloid or the spleen. **High-Yield Facts for NEET-PG:** * **Staining:** Amyloid shows **Apple-green birefringence** under polarized light when stained with **Congo Red** [1]. * **Most common organ involved in Systemic Amyloidosis:** Kidney (most common cause of death). * **Most common organ involved in Secondary Amyloidosis:** Spleen. * **Mnemonic:** **S**ago = **S**plenic follicles (**W**hite pulp); **L**ardaceous = **S**plenic sinusoids (**R**ed pulp). [1] **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 268-269.

Question 660: Premutation is seen in which of the following conditions?

A. Genomic imprinting
B. Trinucleotide repeat mutation (Correct Answer)
C. Mitochondrial mutation
D. Gonadal mosaicism

Explanation: **Explanation:** **Trinucleotide repeat mutations** (Option B) [1] are characterized by the expansion of specific three-nucleotide sequences [2]. The term **"Premutation"** refers to an intermediate number of repeats that does not cause a clinical phenotype in the individual but is highly unstable [1]. During gametogenesis, these premutation alleles tend to expand significantly into a "full mutation," leading to symptomatic disease in the offspring [1]. This phenomenon is the molecular basis for **Anticipation** (earlier onset and increased severity in successive generations). A classic example is **Fragile X Syndrome**, where 55–200 CGG repeats constitute a premutation, while >200 repeats result in the full clinical syndrome [1]. **Why other options are incorrect:** * **Genomic Imprinting (A):** Refers to the epigenetic silencing of one parental allele (e.g., Prader-Willi/Angelman syndromes) [2]. It involves methylation, not repeat expansion. * **Mitochondrial Mutation (C):** Follows maternal inheritance patterns and exhibits **heteroplasmy** (variable mixture of mutant and wild-type DNA), but does not involve premutation stages [2]. * **Gonadal Mosaicism (D):** Occurs when a mutation is present only in a subset of germ cells [2]. It explains how healthy parents can have multiple children with autosomal dominant conditions (e.g., Osteogenesis Imperfecta). **High-Yield Clinical Pearls for NEET-PG:** * **Fragile X Syndrome:** Most common inherited cause of intellectual disability; associated with *FMR1* gene (CGG repeats) [1]. * **Huntington Disease:** CAG repeats; shows paternal expansion bias. * **Myotonic Dystrophy:** CTG repeats; shows maternal expansion bias. * **Friedreich Ataxia:** GAA repeats; notably **Autosomal Recessive** (unlike most other triplet repeats). **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 179-181. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 177.

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Adaptations of Cellular Growth

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Hemodynamic Disorders

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Genetic Disorders

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Environmental Pathology

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Molecular Basis of Disease

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