Genetic Disorders and Biochemical Pathology — MCQs

A female infant appeared normal at birth but developed signs of liver disease and muscular weakness at 3 months. She had periods of hypoglycemia, particularly on awakening. Examination revealed hepatomegaly. Laboratory analyses following fasting revealed ketoacidosis, blood pH 7.25, and elevations in both alanine transaminase (ALT) and aspartate transaminase (AST). Administration of glucagon following a carbohydrate meal elicited a normal rise in blood glucose, but glucose levels did not rise when glucagon was administered following an overnight fast. Liver biopsy revealed an increase in the glycogen content. To prevent the frequent episodes of hypoglycemia, which of the following dietary supplements would be most appropriate for this patient?

Q86Easy

Maternal disomy of chromosome 15 is seen in which of the following conditions?

Q87Easy

Hypophosphatemic vitamin D-resistant rickets is which type of genetic disorder?

Q88Easy

Which disorder affects the muscles?

Q89Easy

Red or brown discoloration of teeth is seen in which condition?

Q90Medium

MELAS is an inherited condition which occurs due to deficiency of which mitochondrial respiratory chain complex?

Genetic Disorders and Biochemical Pathology Indian Medical PG Practice Questions and MCQs

Question 81: A patient presents with skin bullae upon sun exposure. A defect in which of the following is responsible for this condition?

A. Thymidine dimers (Correct Answer)
B. Trinucleotide repeats
C. Sugar changes
D. DNA methylation

Explanation: ### Explanation The clinical presentation of **skin bullae (blisters) upon sun exposure** is characteristic of **Xeroderma Pigmentosum (XP)**, an autosomal recessive genetic disorder. **1. Why Thymidine Dimers is Correct:** Ultraviolet (UV) radiation from sunlight causes adjacent pyrimidine bases (usually two thymines) in DNA to form abnormal covalent bonds, known as **Thymidine Dimers**. In healthy individuals, these are repaired by the **Nucleotide Excision Repair (NER)** pathway. Patients with XP have a genetic defect in the NER enzymes (specifically UV-specific endonucleases). This leads to an accumulation of unrepaired thymidine dimers, causing DNA mutations, cell death (blistering), and a significantly high risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **2. Why the Other Options are Incorrect:** * **Trinucleotide repeats:** These are associated with "Dynamic Mutations" seen in disorders like Huntington’s disease, Fragile X syndrome, and Friedreich’s ataxia, which do not involve photosensitivity. * **Sugar changes:** Alterations in the deoxyribose sugar backbone are not the primary mechanism of UV-induced damage or XP. * **DNA methylation:** This is an epigenetic modification involved in gene silencing (e.g., Genomic Imprinting disorders like Prader-Willi or Angelman syndromes) and is not triggered by UV light. **Clinical Pearls for NEET-PG:** * **Key Enzyme Defect:** UV-specific endonuclease (excision nuclease). * **Early Signs:** Severe sunburn after minimal exposure, "parchment-like" dry skin, and excessive freckling (lentigines) before age 2. * **Associated Condition:** **Cockayne Syndrome** also involves NER defects but presents with "Mickey Mouse" facies and dwarfism without a high risk of skin cancer. * **Diagnostic Test:** Chromosomal breakage study or measuring DNA repair rates in cultured fibroblasts.

Question 82: What is the risk of siblings inheriting Wilson disease from an affected patient?

A. 25% (Correct Answer)
B. 50%
C. 75%
D. 100%

Explanation: **Explanation:** **1. Why 25% is Correct:** Wilson disease is an **Autosomal Recessive (AR)** disorder caused by mutations in the *ATP7B* gene on chromosome 13. In AR inheritance, an affected patient (genotype *aa*) must have inherited one mutant allele from each parent. Therefore, both parents are obligate carriers (genotype *Aa*). When two carriers (*Aa x Aa*) have children, the probability of offspring genotypes according to Mendelian genetics is: * 25% Affected (*aa*) * 50% Carriers (*Aa*) * 25% Genotypically Normal (*AA*) Thus, each sibling of an affected patient has a **25% (1 in 4)** risk of inheriting the disease. **2. Why Other Options are Incorrect:** * **50%:** This would be the risk if the condition were Autosomal Dominant (with one affected parent) or the risk of a sibling being a *carrier* in an AR condition. * **75%:** This represents the probability of a sibling being "unaffected" (Normal + Carrier) in an AR condition. * **100%:** This would only occur if both parents were affected by the same AR disorder. **3. Clinical Pearls for NEET-PG:** * **Defect:** Impaired biliary copper excretion and failure to incorporate copper into apoceruloplasmin. * **Diagnosis:** Low serum Ceruloplasmin, high urinary copper, and **Kayser-Fleischer (KF) rings** (copper deposition in Descemet’s membrane). * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine; Zinc (inhibits intestinal absorption). * **Screening:** Because of the 25% recurrence risk, **first-degree relatives** (especially siblings) of a newly diagnosed patient must be screened even if asymptomatic.

Question 83: Farber's disease is caused by a deficiency in which enzyme?

A. Alpha-galactosidase
B. Ceramidase (Correct Answer)
C. Debranching enzyme
D. None of the above

Explanation: **Explanation:** **Farber’s disease** (also known as Farber’s lipogranulomatosis) is a rare autosomal recessive lysosomal storage disorder. It is caused by a deficiency of the enzyme **acid ceramidase**. 1. **Why Ceramidase is correct:** Acid ceramidase is responsible for breaking down **ceramide** into sphingosine and a free fatty acid within the lysosomes. When this enzyme is deficient, ceramide accumulates in various tissues, particularly in the joints, skin, and central nervous system. This leads to the classic clinical triad: **painful swollen joints, subcutaneous nodules, and a hoarse cry** (due to laryngeal involvement). 2. **Why other options are incorrect:** * **Alpha-galactosidase:** Deficiency of Alpha-galactosidase A causes **Fabry disease**, characterized by angiokeratomas, peripheral neuropathy, and renal failure. * **Debranching enzyme:** Deficiency of this enzyme (Amylo-1,6-glucosidase) causes **Cori disease (GSD Type III)**, a glycogen storage disorder presenting with hepatomegaly and hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "Farber’s is **C**eramide **A**ccumulation" (Farber = **C**eramidase). * **Pathognomonic finding:** Biopsy of nodules shows "Farber bodies" (comma-shaped structures or curvilinear bodies) within macrophages. * **Key Triad:** Arthritis (joint deformity), Subcutaneous nodules, and Hoarseness of voice. * Farber’s disease is unique among sphingolipidoses for its prominent involvement of the joints and larynx.

Question 84: Lysosomal storage disorders are associated with which of the following?

A. Non-disease-specific physical findings
B. Loss of function (Correct Answer)
C. Enzyme assays not useful
D. Genetic testing is helpless

Explanation: **Explanation:** **Lysosomal Storage Disorders (LSDs)** are a group of approximately 50 inherited metabolic diseases characterized by the abnormal accumulation of macromolecules within lysosomes. **Why "Loss of Function" is Correct:** The fundamental pathophysiology of LSDs is a **loss-of-function mutation** in genes encoding specific lysosomal hydrolases, transport proteins, or enzyme activators. This deficiency leads to the inability to degrade specific substrates (like glycosaminoglycans, sphingolipids, or glycogen). The resulting "storage" of undigested material causes cellular dysfunction, organomegaly, and progressive multi-system damage. **Analysis of Incorrect Options:** * **A. Non-disease-specific physical findings:** Incorrect. While some symptoms overlap (e.g., hepatosplenomegaly), many LSDs have **pathognomonic** findings, such as the "Cherry-red spot" on the macula (Tay-Sachs, Niemann-Pick) or "Gaucher cells" (wrinkled paper appearance) in bone marrow. * **C. Enzyme assays not useful:** Incorrect. Enzyme assays are the **gold standard** for definitive biochemical diagnosis. Measuring the activity of the specific deficient enzyme in leukocytes or fibroblasts is the primary diagnostic step. * **D. Genetic testing is helpless:** Incorrect. Genetic testing is essential for identifying specific mutations, carrier screening, prenatal diagnosis, and determining eligibility for targeted therapies like Enzyme Replacement Therapy (ERT). **NEET-PG High-Yield Pearls:** * **Inheritance:** Most LSDs are **Autosomal Recessive**, except for **Hunter Syndrome** and **Fabry Disease**, which are **X-linked Recessive**. * **Gaucher Disease:** The most common LSD; characterized by glucocerebrosidase deficiency. * **I-Cell Disease:** Caused by a failure to tag enzymes with **Mannose-6-Phosphate**, leading to enzymes being secreted extracellularly rather than reaching the lysosome.

Question 85: A female infant appeared normal at birth but developed signs of liver disease and muscular weakness at 3 months. She had periods of hypoglycemia, particularly on awakening. Examination revealed hepatomegaly. Laboratory analyses following fasting revealed ketoacidosis, blood pH 7.25, and elevations in both alanine transaminase (ALT) and aspartate transaminase (AST). Administration of glucagon following a carbohydrate meal elicited a normal rise in blood glucose, but glucose levels did not rise when glucagon was administered following an overnight fast. Liver biopsy revealed an increase in the glycogen content. To prevent the frequent episodes of hypoglycemia, which of the following dietary supplements would be most appropriate for this patient?

A. Casein (milk protein)
B. Uncooked cornstarch (Correct Answer)
C. Fructose
D. Lactose

Explanation: ### Explanation **Diagnosis: Glycogen Storage Disease (GSD) Type III (Cori Disease)** The patient presents with hepatomegaly, hypoglycemia, and muscular weakness. The key biochemical clue is the **glucagon stimulation test**: a normal rise in glucose after a meal (when liver glycogen is present) but no rise after fasting (when the body must mobilize glucose from glycogen branches). This indicates a defect in glycogen breakdown. Unlike GSD Type I (Von Gierke), GSD Type III involves **elevated transaminases (AST/ALT)** and **muscular weakness**, as the enzyme deficiency (Debranching enzyme) affects both liver and muscle. **Why Uncooked Cornstarch is Correct:** Uncooked cornstarch is a complex carbohydrate that is digested very slowly by the body. It acts as a **"slow-release" glucose polymer**, providing a steady supply of exogenous glucose over several hours. This prevents the blood glucose levels from dropping during fasting periods (like sleep), thereby bypassing the need for glycogenolysis and preventing hypoglycemic seizures and ketoacidosis. **Why Other Options are Incorrect:** * **Casein (A):** While a high-protein diet is often recommended in GSD III to provide substrates for gluconeogenesis, it does not provide the immediate, sustained glucose release required to prevent acute fasting hypoglycemia as effectively as cornstarch. * **Fructose (C) & Lactose (D):** These sugars require intact metabolic pathways to be converted to glucose. In many GSDs, simple sugars can worsen the metabolic burden or lead to increased lactic acid/glycogen storage without solving the fasting glucose deficit. **Clinical Pearls for NEET-PG:** * **GSD Type I vs. III:** Both have hepatomegaly and hypoglycemia. However, **GSD III** has **elevated transaminases** and **muscle involvement**, but **normal lactate** and **normal uric acid** levels (unlike Type I). * **Enzyme Defect in GSD III:** Debranching enzyme (α-1,6-glucosidase). * **Glucagon Test:** If glucose rises after a meal but not after fasting, think GSD Type III. If it doesn't rise in either state, think GSD Type I.

Question 86: Maternal disomy of chromosome 15 is seen in which of the following conditions?

A. Prader-Willi syndrome (Correct Answer)
B. Klinefelter's syndrome
C. Angelman syndrome
D. Turner's syndrome

Explanation: **Explanation:** The correct answer is **Prader-Willi syndrome (PWS)**. This condition is a classic example of **genomic imprinting**, where the expression of a gene depends on whether it is inherited from the mother or the father. 1. **Why Prader-Willi Syndrome is correct:** PWS occurs due to the **loss of paternal expression** on chromosome **15q11-q13**. This can happen via three mechanisms: * **Paternal Deletion (70%):** Deletion of the active paternal alleles. * **Maternal Uniparental Disomy (UPD) (25-30%):** The child inherits two copies of chromosome 15 from the mother and **none from the father**. Since the maternal copies are normally "silenced" (imprinted), the child lacks the necessary active genes. * **Imprinting defects.** 2. **Why the other options are incorrect:** * **Angelman Syndrome:** This is the "mirror image" of PWS. It is caused by the **loss of maternal expression** (specifically the *UBE3A* gene) on chromosome 15. It most commonly occurs due to **maternal deletion** or **paternal uniparental disomy**. * **Klinefelter’s Syndrome (47, XXY):** A sex chromosome aneuploidy characterized by an extra X chromosome in males. It is not related to chromosome 15 or imprinting. * **Turner’s Syndrome (45, XO):** A sex chromosome monosomy in females. **High-Yield Clinical Pearls for NEET-PG:** * **Prader-Willi Presentation:** Neonatal hypotonia, hyperphagia (leading to early-onset obesity), intellectual disability, and hypogonadism. * **Angelman Presentation:** "Happy Puppet" syndrome—inappropriate laughter, seizures, ataxia, and severe intellectual disability. * **Mnemonic:** **P**ader-Willi = **P**aternal deletion / **M**aternal Disomy. **A**ngelman = **M**aternal deletion / **P**aternal Disomy.

Question 87: Hypophosphatemic vitamin D-resistant rickets is which type of genetic disorder?

A. Autosomal recessive
B. Autosomal dominant
C. X-linked dominant (Correct Answer)
D. X-linked recessive

Explanation: **Explanation:** **Hypophosphatemic Vitamin D-Resistant Rickets (XLH)** is primarily caused by a mutation in the **PHEX gene** (Phosphate-regulating gene with Homologies to Endopeptidases on the X chromosome). 1. **Why X-linked Dominant is correct:** The PHEX gene is located on the **X chromosome**. Because the inheritance pattern is **dominant**, a single mutated allele is sufficient to cause the disease. This means both males (XY) and females (XX) are affected, though females may sometimes show milder symptoms due to X-inactivation (Lyonization). A key pedigree feature is that an affected father will pass the condition to **all of his daughters** but **none of his sons**. 2. **Why other options are incorrect:** * **Autosomal Recessive/Dominant:** While rare forms of hypophosphatemic rickets exist (e.g., ADHR due to FGF23 mutations), the classic "Vitamin D-resistant rickets" referred to in exams is the X-linked variety. * **X-linked Recessive:** In recessive conditions (like Hemophilia), females are typically asymptomatic carriers. In XLH, females are clinically affected, confirming a dominant pattern. **Clinical Pearls for NEET-PG:** * **Pathophysiology:** Mutation leads to increased levels of **FGF-23** (a phosphatonin), which causes renal phosphate wasting and inhibits the 1-alpha-hydroxylase enzyme. * **Biochemical Profile:** Low serum phosphate, normal serum calcium, and **inappropriately low or normal 1,25-(OH)₂D levels** despite rickets. * **Key Feature:** Unlike nutritional rickets, it does **not** respond to physiological doses of Vitamin D. Treatment requires oral phosphate and calcitriol. * **High-Yield Association:** It is the most common heritable form of rickets.

Question 88: Which disorder affects the muscles?

A. Glycogen storage disorders (Correct Answer)
B. Mucopolysaccharidoses
C. Xanthomatosis
D. None of the above

Explanation: **Explanation:** **Glycogen Storage Disorders (GSDs)** are a group of inherited metabolic diseases characterized by deficiencies in enzymes responsible for glycogen synthesis or breakdown. Several types specifically target skeletal and cardiac muscles because these tissues rely heavily on glycogen for energy during exercise or stress. * **Type II (Pompe Disease):** Acid maltase deficiency leads to lysosomal glycogen accumulation, causing severe cardiomyopathy and muscular hypotonia. * **Type V (McArdle Disease):** Myophosphorylase deficiency prevents glycogen breakdown in muscles, leading to exercise intolerance, muscle cramps, and myoglobinuria. **Why other options are incorrect:** * **Mucopolysaccharidoses (MPS):** These are lysosomal storage disorders caused by the inability to degrade glycosaminoglycans (GAGs). They primarily affect connective tissues, bones (dysostosis multiplex), and the CNS, leading to organomegaly and coarse facial features, rather than primary muscle pathology. * **Xanthomatosis:** This refers to the deposition of cholesterol-rich material in tendons or skin, typically associated with hyperlipidemias (e.g., Familial Hypercholesterolemia). It is a lipid metabolism disorder, not a primary muscle disorder. **High-Yield Clinical Pearls for NEET-PG:** * **McArdle Disease (Type V):** Look for the "Second Wind Phenomenon" and a flat lactate curve during the ischemic forearm exercise test. * **Pompe Disease (Type II):** It is the only GSD that is also a **Lysosomal Storage Disorder**. * **Cori Disease (Type III):** Affects both liver and muscle (debranching enzyme deficiency), presenting with hepatomegaly and distal muscle wasting.

Question 89: Red or brown discoloration of teeth is seen in which condition?

A. Porphyria (Correct Answer)
B. Internal resorption
C. Nasmyth's membrane
D. Silver impregnation

Explanation: **Explanation:** **Correct Option: A. Porphyria** The discoloration of teeth in Porphyria, specifically **Congenital Erythropoietic Porphyria (Gunther’s disease)**, is known as **Erythrodontia**. This occurs due to the excessive accumulation and deposition of **uroporphyrin I and coproporphyrin I** in the calcium phosphate of the enamel and dentin during tooth formation. Under ultraviolet (Wood’s) light, these teeth exhibit a characteristic **reddish-pink fluorescence**. **Incorrect Options:** * **B. Internal Resorption:** This is a pathological process where the pulp tissue resorbs the dentin from within. It often results in a localized "Pink spot of Mummery," but it does not cause generalized red/brown discoloration of all teeth. * **C. Nasmyth’s Membrane:** Also known as the primary enamel cuticle, this is a thin residue of the enamel organ. It can sometimes take up extrinsic stains (green or orange) from food or bacteria in children but is not associated with intrinsic red/brown pathology. * **D. Silver Impregnation:** Exposure to silver (argyria) typically results in a **slate-grey or black** discoloration of tissues, not red or brown. **High-Yield Clinical Pearls for NEET-PG:** * **Gunther’s Disease (CEP):** Deficiency of **Uroporphyrinogen III synthase**. It is characterized by extreme photosensitivity, blistering, scarring, and erythrodontia. * **Tetracycline Staining:** Causes yellowish-brown discoloration; these teeth also fluoresce (bright yellow) under UV light. * **Fluorosis:** Causes "mottled enamel" with chalky white patches or brownish staining depending on the severity. * **Alkaptonuria:** While it causes dark urine (ochronosis), it typically stains cartilage and sclera, not primarily the teeth.

Question 90: MELAS is an inherited condition which occurs due to deficiency of which mitochondrial respiratory chain complex?

A. Complex I (Correct Answer)
B. Complex II
C. Complex III
D. All the above

Explanation: **Explanation:** **MELAS** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes) is a multisystem mitochondrial disorder. The primary biochemical defect in MELAS is a deficiency in **Complex I (NADH: ubiquinone oxidoreductase)** of the mitochondrial respiratory chain. 1. **Why Complex I is correct:** Approximately 80% of MELAS cases are caused by a point mutation in the mitochondrial DNA (mtDNA) at position **A3243G**, which affects the **tRNA-Leu (UUR)** gene. This mutation impairs mitochondrial protein synthesis, specifically affecting the subunits of Complex I. This leads to a failure in the electron transport chain, decreased ATP production, and a shift toward anaerobic metabolism, resulting in the characteristic lactic acidosis. 2. **Why other options are incorrect:** * **Complex II:** Unlike other complexes, Complex II (Succinate dehydrogenase) is entirely encoded by **nuclear DNA**, not mtDNA. It is rarely the primary site of defect in classic MELAS. * **Complex III:** While some mitochondrial diseases involve Complex III, it is not the hallmark deficiency associated with the clinical presentation of MELAS. * **All of the above:** While severe mitochondrial dysfunction can eventually affect the entire chain, the specific diagnostic association for MELAS is Complex I. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Stroke-like episodes (usually before age 40), Encephalopathy (seizures/dementia), and Lactic Acidosis. * **Genetics:** Shows **Maternal Inheritance** and **Heteroplasmy** (variable severity based on the ratio of mutant to normal mtDNA). * **Diagnosis:** Muscle biopsy shows **"Ragged Red Fibers"** (Gomori trichrome stain) due to compensatory subsarcolemmal mitochondrial proliferation. * **Biochemical Marker:** Elevated serum and CSF lactate levels.

Practice by Chapter

Single Gene Disorders

Practice Questions

Biochemical Diagnosis of Genetic Disorders

Practice Questions

Inborn Errors of Metabolism

Practice Questions

Lysosomal Storage Diseases

Practice Questions

Glycogen Storage Diseases

Practice Questions

Disorders of Lipoprotein Metabolism

Practice Questions

Disorders of Purine and Pyrimidine Metabolism

Practice Questions

Hemoglobinopathies

Practice Questions

Porphyrias

Practice Questions

Biochemical Markers for Disease Diagnosis

Practice Questions

Newborn Screening for Genetic Disorders

Practice Questions

Enzyme Replacement Therapy

Practice Questions

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