Genetic Disorders and Biochemical Pathology — MCQs
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A woman who suffers from frequent and severe migraine headaches has had five children, all of whom have experienced, beginning between the ages of 8 and 12, stroke-like episodes compounded with exercise intolerance and lactic acidosis. The father of the children does not suffer from migraines, nor is he exercise intolerant. A target of a mutation that can explain these findings is most likely which one of the following?
The gene for Wilson's disease is located on which chromosome?
Which chromosome is associated with cystic fibrosis?
An infant is unable to feed properly, is weak, and not gaining weight. The mother reports multiple episodes of urination with crying each time the baby passes urine. She also notes that the baby often smells of rotten fish in his urine and sweat. Which of the following would you test for in the infant's urine?
Which of the following characterizes Gilbert's syndrome?
Chromosome 7 involvement is seen in which of the following conditions?
A 9-month-old infant presented with recurrent infections. Investigations revealed a near absence of B and T cells and a significantly diminished thymic shadow on chest X-ray. Which of the following metabolites would be elevated in this patient?
For which of the following diseases is enzyme replacement therapy available?
Von Gierke's disease occurs due to deficiency of which enzyme?
Barth syndrome is due to a defect in which of the following?
Genetic Disorders and Biochemical Pathology Indian Medical PG Practice Questions and MCQs
Question 251: A woman who suffers from frequent and severe migraine headaches has had five children, all of whom have experienced, beginning between the ages of 8 and 12, stroke-like episodes compounded with exercise intolerance and lactic acidosis. The father of the children does not suffer from migraines, nor is he exercise intolerant. A target of a mutation that can explain these findings is most likely which one of the following?
- A. Mitochondrial tRNA (Correct Answer)
- B. Cytoplasmic tRNA
- C. Cytochrome c
- D. Pyruvate dehydrogenase
Explanation: ### Explanation **1. Why Mitochondrial tRNA is Correct:** The clinical presentation describes **MELAS syndrome** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes). The key diagnostic features in the stem are: * **Maternal Inheritance:** The mother is affected (migraines are a common phenotypic expression), and *all* five children are affected, while the father is healthy. This pattern is characteristic of mitochondrial DNA (mtDNA) mutations. * **Clinical Triad:** Stroke-like episodes (often before age 40), lactic acidosis, and exercise intolerance. * **Molecular Basis:** Approximately 80% of MELAS cases are caused by a point mutation in the **MT-TL1 gene**, which encodes the **mitochondrial tRNA-Leucine (UUR)**. This impairs mitochondrial protein synthesis, leading to a deficiency in respiratory chain complexes. **2. Why Incorrect Options are Wrong:** * **B. Cytoplasmic tRNA:** These are encoded by nuclear DNA. Mutations would follow Mendelian inheritance (Autosomal/X-linked) and would not specifically target mitochondrial oxidative phosphorylation. * **C. Cytochrome c:** While part of the electron transport chain, it is a protein. While some ETC components are mtDNA-encoded, the classic "stroke-like episode" presentation is specifically linked to tRNA mutations. * **D. Pyruvate dehydrogenase (PDH):** PDH deficiency causes lactic acidosis and neurological delay, but it follows **X-linked inheritance** (most common form) and does not typically present with the specific "stroke-like episodes" or the 100% maternal transmission seen here. **3. Clinical Pearls for NEET-PG:** * **Heteroplasmy:** The severity of mitochondrial diseases varies among siblings due to the random distribution of mutated vs. normal mitochondria during oogenesis. * **Muscle Biopsy:** Look for **"Ragged Red Fibers"** (Gomori trichrome stain) representing compensatory mitochondrial proliferation. * **Maternal Inheritance Rule:** An affected mother transmits to **all** children; an affected father transmits to **none**.
Question 252: The gene for Wilson's disease is located on which chromosome?
- A. Long arm of Chromosome 13 (Correct Answer)
- B. Long arm of Chromosome 6
- C. Short arm of Chromosome 13
- D. Short arm of Chromosome 6
Explanation: **Explanation:** **Wilson’s Disease (Hepatolenticular Degeneration)** is an autosomal recessive disorder of copper metabolism. The correct answer is **Option A (Long arm of Chromosome 13)** because the disease is caused by a mutation in the **ATP7B gene**, which is located on chromosome **13q14.3**. This gene encodes a P-type ATPase copper-transporting protein responsible for transporting copper into the bile and incorporating it into apoceruloplasmin to form ceruloplasmin. A defect leads to toxic copper accumulation in the liver, brain (basal ganglia), and cornea. **Analysis of Incorrect Options:** * **Option B & D (Chromosome 6):** Chromosome 6 is the site of the **HFE gene** (specifically 6p21.3), which is mutated in **Hereditary Hemochromatosis** (iron overload). * **Option C (Short arm of Chromosome 13):** The ATP7B gene is specifically located on the **long arm (q)**, not the short arm (p). In genetics, "q" stands for the long arm (queue) and "p" for the short arm (petit). **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Markers:** Low serum ceruloplasmin, increased urinary copper excretion, and increased hepatic copper content. * **Ocular Sign:** **Kayser-Fleischer (KF) rings** due to copper deposition in the Descemet’s membrane of the cornea. * **Neurological Sign:** "Wing-beating" tremor and parkinsonian features. * **Diagnosis:** Liver biopsy is the gold standard; "Giant Panda" sign on MRI brain. * **Treatment:** Copper chelators like **D-Penicillamine** (first-line) or Trientine, and Zinc (to prevent absorption).
Question 253: Which chromosome is associated with cystic fibrosis?
- A. 7 (Correct Answer)
- B. 8
- C. 11
- D. 13
Explanation: **Explanation:** **1. Correct Answer: Option A (Chromosome 7)** Cystic Fibrosis (CF) is an **autosomal recessive** multisystem disorder caused by mutations in the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene**, which is located on the **long arm (q) of Chromosome 7** (specifically at position 7q31.2). The CFTR protein functions as a cAMP-regulated chloride channel. The most common mutation is the **ΔF508** (deletion of phenylalanine at position 508), which leads to protein misfolding and degradation in the endoplasmic reticulum. **2. Analysis of Incorrect Options:** * **Option B (Chromosome 8):** Associated with conditions like **Hereditary Spherocytosis** (ANK1 gene) and Myc oncogene translocations (Burkitt Lymphoma). * **Option C (Chromosome 11):** A high-yield chromosome containing genes for **Beta-globin** (Sickle cell anemia, Beta-thalassemia), Insulin, and the WT1 gene (Wilms tumor). * **Option D (Chromosome 13):** Associated with **Patau Syndrome** (Trisomy 13), **Wilson disease** (ATP7B gene), and **Retinoblastoma** (RB1 gene). **3. Clinical Pearls for NEET-PG:** * **Diagnosis:** The gold standard is the **Pilocarpine Iontophoresis Sweat Chloride Test** (Chloride >60 mEq/L is diagnostic). * **Clinical Presentation:** Recurrent pulmonary infections (*Pseudomonas* is common), pancreatic insufficiency (steatorrhea), and **meconium ileus** in newborns. * **Infertility:** 95% of males are infertile due to **Congenital Bilateral Absence of Vas Deferens (CBAVD)**. * **Biochemical Hallmark:** Defective chloride transport leads to thick, dehydrated secretions in the lungs and pancreas.
Question 254: An infant is unable to feed properly, is weak, and not gaining weight. The mother reports multiple episodes of urination with crying each time the baby passes urine. She also notes that the baby often smells of rotten fish in his urine and sweat. Which of the following would you test for in the infant's urine?
- A. Viny/mandelic acid
- B. Isovaleric acid
- C. Trimethylamine (Correct Answer)
- D. Oxoisocaproic acid
Explanation: ### Explanation **Correct Option: C. Trimethylamine** The clinical presentation of a "rotten fish" odor in the urine and sweat is the pathognomonic sign of **Trimethylaminuria**, also known as **Fish Odor Syndrome**. This condition is caused by a deficiency of the enzyme **Flavin-containing monooxygenase 3 (FMO3)**. Normally, FMO3 converts trimethylamine (TMA)—a malodorous byproduct of bacterial degradation of choline, lecithin, and carnitine in the gut—into the odorless **trimethylamine N-oxide (TMAO)**. When FMO3 is deficient, TMA accumulates and is excreted through sweat, urine, and breath, resulting in the characteristic fishy smell. The infant’s failure to thrive and distress during urination (likely due to skin irritation or systemic malaise) are consistent with severe metabolic presentations. **Analysis of Incorrect Options:** * **A. Vinylmandelic acid (VMA):** This is a urinary metabolite of catecholamines (epinephrine/norepinephrine). It is used to diagnose **Neuroblastoma** in children or Pheochromocytoma in adults, but it does not cause a fishy odor. * **B. Isovaleric acid:** This accumulates in **Isovaleric Acidemia**. While it causes a distinct odor, it is described as **"sweaty feet"** or "cheesy" rather than rotten fish. * **D. Oxoisocaproic acid:** This is a branched-chain keto acid that accumulates in **Maple Syrup Urine Disease (MSUD)**. The characteristic odor associated with this is **burnt sugar** or maple syrup. **High-Yield Clinical Pearls for NEET-PG:** * **Trimethylaminuria (FMO3 deficiency):** Autosomal recessive; management involves dietary restriction of precursors (fish, eggs, legumes). * **Odor Associations:** * **Mousy/Musty:** Phenylketonuria (PKU). * **Cabbage-like/Rancid butter:** Tyrosinemia Type I. * **Sweaty feet:** Isovaleric acidemia or Glutaric aciduria type II. * **Swimming pool odor:** Hawkinsinuria.
Question 255: Which of the following characterizes Gilbert's syndrome?
- A. Normal liver function tests
- B. Increased risk of cirrhosis
- C. Normal liver histology (Correct Answer)
- D. Conjugated hyperbilirubinemia precipitated by fasting
Explanation: **Explanation:** **Gilbert’s Syndrome** is the most common hereditary cause of hyperbilirubinemia, affecting approximately 3–7% of the population. It is caused by a genetic mutation (typically a TATAA box polymorphism) in the **UGT1A1 gene**, leading to a reduction (about 30% of normal) in the activity of the enzyme **UDP-glucuronosyltransferase**. 1. **Why "Normal liver histology" is correct:** Gilbert’s syndrome is a benign condition. Since the pathology is limited to a mild functional deficiency in bilirubin conjugation, there is no structural damage to the hepatocytes or the biliary architecture. Under a microscope, the liver appears completely normal. 2. **Why the other options are incorrect:** * **Option A:** While most LFTs (ALT, AST, Alkaline Phosphatase) are normal, the **total bilirubin is elevated** (predominantly unconjugated). Therefore, saying "all" LFTs are normal is technically inaccurate in a clinical context. * **Option B:** Gilbert’s syndrome is a benign condition and does **not** progress to inflammation, fibrosis, or cirrhosis. * **Option D:** The condition is characterized by **unconjugated** (indirect) hyperbilirubinemia. While it is indeed precipitated by fasting, stress, or illness, the type of bilirubin is unconjugated, not conjugated. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Usually Autosomal Recessive. * **Triggers:** Jaundice is typically mild (<3 mg/dL) and triggered by **fasting (caloric restriction)**, dehydration, physical exertion, or febrile illness. * **Diagnostic Test:** The "Fasting Test" (bilirubin rises significantly after a 48-hour fast) or the "Rifampicin Test." * **Management:** No treatment is required; reassurance is key. * **Differential:** Distinguish from **Crigler-Najjar Syndrome** (more severe UGT1A1 deficiency) and **Dubin-Johnson/Rotor Syndromes** (which present with conjugated hyperbilirubinemia).
Question 256: Chromosome 7 involvement is seen in which of the following conditions?
- A. Osteosarcoma
- B. Osteosarcoma
- C. Cystic fibrosis (Correct Answer)
- D. Colon cancer
Explanation: **Explanation:** **Cystic Fibrosis (Correct Answer):** Cystic Fibrosis (CF) is an autosomal recessive disorder caused by mutations in the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene**, which is located on the **long arm (q) of Chromosome 7** (specifically 7q31.2). The most common mutation is the deletion of phenylalanine at position 508 (**ΔF508**). This defect leads to abnormal chloride transport across epithelial membranes, resulting in thick, viscid secretions in the lungs, pancreas, and reproductive tract. **Incorrect Options:** * **Osteosarcoma:** While complex chromosomal rearrangements occur, the most characteristic genetic associations are with the **RB1 gene (Chromosome 13q)** and the **TP53 gene (Chromosome 17p)**. It is frequently seen in patients with hereditary retinoblastoma or Li-Fraumeni syndrome. * **Colon Cancer:** The primary genetic drivers for colorectal cancer are located on other chromosomes. For example, the **APC gene** (associated with FAP) is on **Chromosome 5q**, and the **DCC gene** is on **Chromosome 18q**. Lynch syndrome (HNPCC) involves DNA mismatch repair genes like MSH2 (Chr 2) and MLH1 (Chr 3). **High-Yield Clinical Pearls for NEET-PG:** * **Chromosome 7 Associations:** Apart from Cystic Fibrosis, Chromosome 7 is also home to the **ELN gene** (mutated in **Williams Syndrome**, specifically a microdeletion at 7q11.23). * **Diagnostic Test:** The gold standard for CF diagnosis is the **Sweat Chloride Test** (Pilocarpine Iontophoresis); chloride levels >60 mEq/L are diagnostic. * **Common Complication:** Recurrent pulmonary infections, most commonly by *Pseudomonas aeruginosa* in older patients and *Staphylococcus aureus* in children.
Question 257: A 9-month-old infant presented with recurrent infections. Investigations revealed a near absence of B and T cells and a significantly diminished thymic shadow on chest X-ray. Which of the following metabolites would be elevated in this patient?
- A. Uric acid
- B. Orotic acid
- C. Deoxyadenosine (Correct Answer)
- D. NADPH
Explanation: ### Explanation The clinical presentation of recurrent infections, near absence of both B and T cells (lymphopenia), and a diminished thymic shadow in a 9-month-old infant is characteristic of **Severe Combined Immunodeficiency (SCID)**. Approximately 15% of SCID cases are caused by a deficiency in the enzyme **Adenosine Deaminase (ADA)**. **Why Deoxyadenosine is Correct:** Adenosine Deaminase is a crucial enzyme in the purine salvage pathway that converts adenosine to inosine and deoxyadenosine to deoxyinosine. In ADA deficiency, **deoxyadenosine** accumulates and is phosphorylated into **dATP (deoxyadenosine triphosphate)**. High levels of dATP are toxic to lymphocytes because they inhibit **Ribonucleotide Reductase**, the enzyme responsible for DNA synthesis. This leads to a failure of T and B cell proliferation, resulting in the SCID phenotype. **Analysis of Incorrect Options:** * **A. Uric Acid:** This is the end product of purine catabolism. In ADA deficiency, the pathway is blocked upstream, typically leading to *decreased* uric acid production. * **B. Orotic Acid:** Elevated in Urea Cycle disorders (like OTC deficiency) or Orotic Aciduria. It is not directly linked to the purine salvage pathway or SCID. * **D. NADPH:** This is a cofactor produced in the HMP shunt. While essential for respiratory burst in neutrophils (deficient in Chronic Granulomatous Disease), it does not accumulate in ADA deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** ADA deficiency is the most common **Autosomal Recessive** cause of SCID (X-linked SCID is the overall most common type). * **First-line Treatment:** Bone marrow transplantation or Enzyme Replacement Therapy (PEG-ADA). * **Gene Therapy:** ADA deficiency was the first genetic disease for which gene therapy was attempted. * **Radiology:** A "missing thymic shadow" in a pediatric chest X-ray should immediately raise suspicion for SCID or DiGeorge Syndrome.
Question 258: For which of the following diseases is enzyme replacement therapy available?
- A. Albinism
- B. Niemann-Pick disease
- C. Metachromatic leukodystrophy
- D. Gaucher's disease (Correct Answer)
Explanation: **Explanation:** **Gaucher’s Disease (Correct Answer):** Gaucher’s disease is a lysosomal storage disorder caused by a deficiency of the enzyme **β-glucocerebrosidase**, leading to the accumulation of glucocerebroside in macrophages. It was the first lysosomal storage disorder for which **Enzyme Replacement Therapy (ERT)** was successfully developed. Recombinant enzymes like **Imiglucerase**, Velaglucerase alfa, and Taliglucerase are the standard of care, significantly reducing hepatosplenomegaly and improving hematological parameters. **Why the other options are incorrect:** * **Albinism:** This is a defect in melanin synthesis (typically due to **Tyrosinase** deficiency). It is not a lysosomal storage disorder and cannot be treated with ERT because the enzyme cannot be targeted effectively to melanocytes via the bloodstream. * **Niemann-Pick Disease:** While ERT (Olipudase alfa) has recently been approved for Niemann-Pick Type B (Acid Sphingomyelinase deficiency), it is not yet the "classic" or widely established answer in standard medical examinations compared to Gaucher’s. Type A involves severe neurodegeneration where ERT cannot cross the blood-brain barrier. * **Metachromatic Leukodystrophy (MLD):** Caused by **Arylsulfatase A** deficiency. While gene therapy (Atidarsagene autotemcel) has shown promise, ERT is not the standard established treatment for MLD in the context of NEET-PG questions. **NEET-PG High-Yield Pearls:** * **Gaucher Cells:** Characterized by a "wrinkled paper" or "crumpled silk" appearance of the cytoplasm. * **Other ERT-treatable diseases:** Fabry’s disease (α-galactosidase A), Pompe disease (α-glucosidase), and Hurler syndrome (Iduronidase). * **Targeting Mechanism:** ERT for lysosomal disorders relies on the **Mannose-6-Phosphate (M6P)** receptor-mediated uptake of the enzyme into the cells.
Question 259: Von Gierke's disease occurs due to deficiency of which enzyme?
- A. Liver phosphorylase
- B. Glucose-6-phosphatase (Correct Answer)
- C. Muscle phosphorylase
- D. Debranching enzyme
Explanation: **Explanation:** **Von Gierke’s Disease (GSD Type I)** is the most common glycogen storage disease. It is caused by a deficiency of **Glucose-6-phosphatase (G6Pase)**, an enzyme primarily located in the liver and kidneys. This enzyme is responsible for the final step of both glycogenolysis and gluconeogenesis: converting Glucose-6-phosphate into free glucose. Without it, the liver cannot release glucose into the bloodstream, leading to severe fasting hypoglycemia and massive hepatomegaly due to glycogen accumulation. **Analysis of Options:** * **Option B (Correct):** Glucose-6-phosphatase deficiency prevents the terminal step of glucose production, causing the characteristic biochemical triad of hypoglycemia, lactic acidosis, and hyperuricemia. * **Option A (Incorrect):** Deficiency of **Liver phosphorylase** causes **Hers disease (GSD Type VI)**, which presents with a milder clinical course than Von Gierke’s. * **Option C (Incorrect):** Deficiency of **Muscle phosphorylase** causes **McArdle disease (GSD Type V)**, characterized by exercise intolerance and muscle cramps, but no hypoglycemia. * **Option D (Incorrect):** Deficiency of the **Debranching enzyme** (α-1,6-glucosidase) causes **Cori disease (GSD Type III)**, where "limit dextrin" accumulates. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Hallmarks:** Hyperlipidemia (doll-like face), Hyperuricemia (Gout), and Lactic Acidosis. * **Diagnostic Clue:** Administration of glucagon or epinephrine does *not* increase blood glucose levels in these patients (unlike in Cori disease). * **Management:** Frequent feedings with uncooked cornstarch to maintain glucose levels and prevent nocturnal hypoglycemia.
Question 260: Barth syndrome is due to a defect in which of the following?
- A. Thermogenic
- B. Cardiolipin (Correct Answer)
- C. Ubiquinone
- D. Cytochrome
Explanation: **Explanation:** **Barth Syndrome** (also known as 3-Methylglutaconic aciduria type II) is an X-linked recessive disorder caused by a mutation in the **TAZ gene**, which encodes the protein **Tafazzin**. Tafazzin is an acyltransferase responsible for the remodeling and maturation of **Cardiolipin**, a unique phospholipid found exclusively in the inner mitochondrial membrane. 1. **Why Cardiolipin is Correct:** Cardiolipin is essential for stabilizing the mitochondrial respiratory chain complexes and maintaining the structural integrity of the cristae. In Barth syndrome, defective remodeling leads to a deficiency of mature cardiolipin, resulting in mitochondrial dysfunction. This manifests clinically as the triad of **dilated cardiomyopathy, skeletal myopathy, and neutropenia.** 2. **Why Other Options are Incorrect:** * **Thermogenin (UCP1):** This is an uncoupling protein found in brown adipose tissue that generates heat. It is not associated with Barth syndrome. * **Ubiquinone (Coenzyme Q10):** This is a mobile electron carrier in the electron transport chain. While CoQ10 deficiency causes mitochondrial diseases, it is not the primary defect in Barth syndrome. * **Cytochrome:** Cytochromes (like Cytochrome c) are heme-containing proteins in the ETC. Mutations in cytochrome oxidase can cause Leigh syndrome, but not Barth syndrome. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked Recessive (affects males). * **Biochemical Marker:** Elevated **3-methylglutaconic acid** in urine. * **Key Clinical Features:** "Cardio-Skeletal-Neutro" (Cardiomyopathy, Myopathy, Neutropenia, and Growth retardation). * **Mnemonic:** Think of **"Barth"** as **"Heart"** (Cardiomyopathy) and **"Cardio"**-lipin.
Practice by Chapter
Single Gene Disorders
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Biochemical Diagnosis of Genetic Disorders
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Inborn Errors of Metabolism
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Lysosomal Storage Diseases
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Glycogen Storage Diseases
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Disorders of Lipoprotein Metabolism
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Disorders of Purine and Pyrimidine Metabolism
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Hemoglobinopathies
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Porphyrias
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Biochemical Markers for Disease Diagnosis
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Newborn Screening for Genetic Disorders
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Enzyme Replacement Therapy
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