Genetic Disorders and Biochemical Pathology — MCQs

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

Question 41: What inborn error of metabolism is characterized by a swimming pool-like odor?

A. Tyrosinemia
B. Hawkinsinuria (Correct Answer)
C. Phenylketonuria
D. Multiple carboxylase deficiency

Explanation: **Explanation:** **Hawkinsinuria** is a rare autosomal dominant metabolic disorder involving the metabolism of tyrosine. It is caused by a deficiency in the enzyme **4-hydroxyphenylpyruvate dioxygenase (4-HPPD)**. This defect leads to the accumulation of an intermediate metabolite called **Hawkinsin**. The characteristic clinical hallmark of this condition is a distinct **swimming pool-like odor** (chlorine-like) of the urine and sweat. Infants typically present with failure to thrive and metabolic acidosis when transitioning to high-protein diets (like breast milk or formula). **Analysis of Incorrect Options:** * **Tyrosinemia (Type I):** Characterized by a **boiled cabbage** or rancid butter odor due to the accumulation of succinylacetone. * **Phenylketonuria (PKU):** Characterized by a **mousy or musty odor** due to the accumulation of phenylacetic acid. * **Multiple Carboxylase Deficiency:** Often associated with a **tomcat urine** odor (similar to Isovaleric acidemia, which is "sweaty feet"). **NEET-PG High-Yield Pearls (Odors in Metabolic Disorders):** * **Maple Syrup Urine Disease (MSUD):** Burnt sugar/Maple syrup odor (Isoleucine). * **Isovaleric Acidemia / Glutaric Acidemia Type II:** Sweaty feet odor. * **Trimethylaminuria:** Fishy odor. * **Hypermethioninemia:** Boiled cabbage odor. * **Oasthouse Urine Disease:** Dried malt or hops odor. **Key Takeaway:** For NEET-PG, associate **Hawkinsinuria** specifically with the **"Swimming Pool/Chlorine"** odor and a defect in **Tyrosine** catabolism.

Question 42: Which of the following diseases is NOT a mitochondrial disorder?

A. Leigh syndrome
B. Leber's hereditary optic neuropathy
C. Myoclonic epilepsy
D. Huntington's disease (Correct Answer)

Explanation: **Explanation:** The correct answer is **Huntington’s disease** because it is an **autosomal dominant** neurodegenerative disorder caused by a trinucleotide repeat expansion (CAG) on Chromosome 4. It is a nuclear DNA defect, not a mitochondrial DNA (mtDNA) defect. **Why the other options are Mitochondrial Disorders:** Mitochondrial diseases typically involve defects in oxidative phosphorylation and exhibit **maternal inheritance** (mitochondrial inheritance). * **Leigh Syndrome:** A severe neurological disorder (subacute necrotizing encephalomyelopathy) often caused by mutations in mtDNA-encoded subunits of the electron transport chain (though nuclear mutations can also occur). * **Leber’s Hereditary Optic Neuropathy (LHON):** A classic example of mitochondrial inheritance characterized by bilateral painless loss of central vision due to mutations in NADH dehydrogenase subunits. * **Myoclonic Epilepsy with Ragged Red Fibers (MERRF):** Caused by a mutation in the mitochondrial tRNA-Lysine gene. It is characterized by myoclonus, ataxia, and the presence of "ragged red fibers" (clumps of diseased mitochondria) on Gomori trichrome stain. **NEET-PG High-Yield Pearls:** 1. **Maternal Inheritance:** Mitochondrial disorders are passed only from mothers to all children. Affected fathers do not pass the trait. 2. **Heteroplasmy:** The coexistence of mutated and wild-type mtDNA within a single cell, explaining the variable clinical severity. 3. **Tissues Affected:** Organs with high energy demands (Brain, Heart, Skeletal Muscle) are most commonly involved. 4. **Huntington’s Disease Key Fact:** It exhibits **anticipation** (earlier onset in successive generations), especially during paternal transmission.

Question 43: Which of the following enzyme defects is the most commonly inherited metabolic disorder of glycolysis?

A. Glucokinase
B. Hexokinase
C. Phosphofructokinase
D. Pyruvate kinase (Correct Answer)

Explanation: **Explanation:** **Pyruvate Kinase (PK) deficiency** is the most common inherited enzyme defect of the **glycolytic pathway** (Embden-Meyerhof pathway) and the second most common cause of enzyme-deficient hemolytic anemia (after G6PD deficiency). **Why Pyruvate Kinase is correct:** Mature erythrocytes lack mitochondria and depend entirely on anaerobic glycolysis for ATP production. PK catalyzes the final step of glycolysis (Phosphoenolpyruvate to Pyruvate), generating ATP. A deficiency leads to ATP depletion, causing failure of the Na⁺/K⁺-ATPase pumps. This results in the loss of intracellular potassium and water, leading to cell dehydration, "echinocyte" (burr cell) formation, and premature destruction in the spleen (extravascular hemolysis). **Why other options are incorrect:** * **Glucokinase:** Defects are rare and typically associated with MODY (Maturity-Onset Diabetes of the Young) type 2, not a primary hemolytic disorder. * **Hexokinase:** Deficiency is extremely rare; it would be more severe as it affects the very first step of glucose utilization. * **Phosphofructokinase (PFK-1):** Deficiency (Tarui disease/GSD Type VII) primarily affects muscles and causes exercise intolerance, though it can cause mild hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Biochemical Hallmark:** Accumulation of **2,3-Bisphosphoglycerate (2,3-BPG)**. This shifts the oxygen dissociation curve to the **right**, facilitating oxygen unloading to tissues (patients often tolerate anemia better than expected). * **Peripheral Smear:** Presence of **Echinocytes** (Burr cells). * **Diagnosis:** Quantitative assay of PK enzyme activity in RBCs.

Question 44: Friedreich's ataxia is caused by which of the following genetic mechanisms?

A. Point mutation
B. Expanded trinucleotide repeat (Correct Answer)
C. Missense mutation
D. Inversion

Explanation: **Explanation:** **Friedreich's Ataxia (FRDA)** is an autosomal recessive neurodegenerative disorder. The correct answer is **Expanded trinucleotide repeat** because the disease is caused by an unstable expansion of the **GAA** triplet repeat in the first intron of the **FXN gene** on chromosome 9. This expansion leads to transcriptional silencing (epigenetic knockdown) of the gene, resulting in a deficiency of the protein **Frataxin**. Frataxin is essential for mitochondrial iron metabolism; its deficiency leads to iron accumulation, oxidative stress, and impaired ATP production, primarily affecting the dorsal root ganglia, spinocerebellar tracts, and the heart. **Why other options are incorrect:** * **Point mutation & Missense mutation:** While rare point mutations can occur in the FXN gene, they account for less than 5% of cases. The hallmark and primary mechanism (95%+) is the triplet repeat expansion. * **Inversion:** This involves a segment of DNA being reversed end-to-end (e.g., Hemophilia A). It is not the mechanism underlying Friedreich's ataxia. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive (unique among most trinucleotide repeat disorders like Huntington’s or Fragile X, which are Dominant/X-linked). * **Repeat Sequence:** **GAA** (Mnemonic: Friedreich is **GAA**y/GAA). * **Clinical Triad:** Progressive ataxia, HOCM (Hypertrophic Obstructive Cardiomyopathy—the most common cause of death), and Diabetes Mellitus. * **Skeletal findings:** Kyphoscoliosis and Pes cavus (high-arched feet).

Question 45: A 3-day-old infant vomits everything they feed, has a distended abdomen, and diarrhea. The urine is positive for reducing substances by Benedict's test. What is the substance in the urine?

A. Sucrose
B. Glucose
C. Galactose (Correct Answer)
D. Fructose

Explanation: ### Explanation The clinical presentation of a 3-day-old neonate with vomiting, abdominal distension, and diarrhea immediately after starting milk feeds, coupled with **reducing substances in the urine**, is a classic description of **Classic Galactosemia** (deficiency of Galactose-1-phosphate uridyltransferase). **1. Why Galactose is correct:** Milk contains lactose, which is disaccharide composed of glucose and galactose. In galactosemia, the infant cannot metabolize galactose. This leads to an accumulation of galactose and galactose-1-phosphate in tissues. Excess galactose is excreted in the urine. Since galactose is a **reducing sugar**, it yields a positive result on **Benedict’s test**. **2. Why other options are incorrect:** * **Sucrose (A):** Sucrose is a **non-reducing sugar** (the only common one) and would give a negative Benedict’s test. Furthermore, sucrose is not found in breast milk. * **Glucose (B):** While glucose is a reducing sugar, isolated glucosuria in a neonate without hyperglycemia is rare and does not typically present with this systemic gastrointestinal distress immediately upon milk ingestion. * **Fructose (D):** Hereditary Fructose Intolerance presents similarly (vomiting, hypoglycemia) but only **after** the introduction of weaning foods (fruit juices/sucrose), not in a 3-day-old on exclusive milk feeds. **3. NEET-PG High-Yield Pearls:** * **Enzyme Deficiency:** Most common is **GALT** (Galactose-1-phosphate uridyltransferase). * **Clinical Triad:** Cataracts (due to **galactitol** accumulation via aldose reductase), hepatosplenomegaly/jaundice, and intellectual disability. * **Infection Risk:** These infants are at high risk for **E. coli sepsis**. * **Diagnosis:** Benedict’s test is positive (nonspecific), but the **Glucose Oxidase test (Dipstick)** is negative (specific for glucose). * **Management:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula.

Question 46: Which gene is involved in the pathogenesis of Type 1 Diabetes mellitus?

A. CTLA-4 (Correct Answer)
B. ABCD1
C. HNF-1 Alpha
D. HNF-4 Alpha

Explanation: **Explanation:** **Correct Answer: A. CTLA-4** Type 1 Diabetes Mellitus (T1DM) is a T-cell-mediated autoimmune destruction of pancreatic beta cells. The **CTLA-4 (Cytotoxic T-Lymphocyte Associated Protein 4)** gene, located on chromosome 2q33, acts as a critical negative regulator of T-cell activation. Polymorphisms in this gene lead to a loss of self-tolerance, allowing autoreactive T-cells to attack islet cells. Other major genetic associations for T1DM include **HLA-DR3/DR4** (the strongest link) and the **PTPN22** gene. **Analysis of Incorrect Options:** * **B. ABCD1:** This gene encodes a peroxisomal membrane transporter. Mutations in *ABCD1* lead to **X-linked Adrenoleukodystrophy**, characterized by the accumulation of very-long-chain fatty acids (VLCFA) in the brain and adrenal glands. * **C. HNF-1 Alpha:** Mutations in the Hepatocyte Nuclear Factor-1 Alpha gene are the most common cause of **MODY Type 3** (Maturity-Onset Diabetes of the Young). * **D. HNF-4 Alpha:** Mutations in this gene cause **MODY Type 1**. Unlike T1DM, MODY is an autosomal dominant monogenic form of diabetes, not an autoimmune process. **High-Yield Clinical Pearls for NEET-PG:** * **HLA Association:** Over 90% of T1DM patients carry HLA-DR3-DQ2 or HLA-DR4-DQ8. * **Autoantibodies:** The presence of GAD65 (Glutamic Acid Decarboxylase), IA-2, and Zinc Transporter 8 (ZnT8) antibodies confirms the autoimmune etiology. * **MODY vs. T1DM:** Suspect MODY in a young, non-obese patient with a strong family history of diabetes (3 generations) and an absence of autoantibodies.

Question 47: All of the following conditions have autosomal dominant inheritance except?

A. Fabry disease (Correct Answer)
B. Marfan's syndrome
C. Osteogenesis imperfecta
D. Ehlers Danlos syndrome

Explanation: **Explanation:** The correct answer is **Fabry disease** because it follows an **X-linked recessive** inheritance pattern, whereas the other options are classic examples of autosomal dominant (AD) disorders. 1. **Fabry Disease (Choice A):** This is a lysosomal storage disorder caused by a deficiency of the enzyme **$\alpha$-galactosidase A**, leading to the accumulation of globotriaosylceramide. Unlike most other sphingolipidoses (which are autosomal recessive), Fabry disease and Hunter syndrome are **X-linked**. Clinical hallmarks include angiokeratomas, peripheral neuropathy (acroparesthesia), hypohidrosis, and progressive renal/cardiac failure. 2. **Marfan’s Syndrome (Choice B):** This is an **AD** connective tissue disorder caused by mutations in the **FBN1 gene** on chromosome 15, which encodes **Fibrillin-1**. It typically presents with tall stature, ectopia lentis, and aortic root dilation. 3. **Osteogenesis Imperfecta (Choice C):** Most common types (Type I-IV) are inherited in an **AD** fashion, resulting from mutations in **COL1A1 or COL1A2** genes. It is characterized by "brittle bones," blue sclera, and hearing loss. 4. **Ehlers-Danlos Syndrome (Choice D):** While EDS is a heterogeneous group, the most common types (like the Classical and Hypermobility types) follow an **AD** inheritance pattern. It involves defects in collagen synthesis or processing, leading to skin hyperextensibility and joint hypermobility. **High-Yield NEET-PG Pearls:** * **Mnemonic for X-linked Recessive:** "**H**unter's **F**abry **G**ames **A**re **C**ool" (**H**unter, **F**abry, **G**6PD, **A**LDP/Adrenoleukodystrophy, **C**GDR/Color blindness). * Most structural protein defects (collagen, fibrillin, spectrin) are **Autosomal Dominant**. * Most enzyme deficiencies (metabolic pathways) are **Autosomal Recessive**, with Fabry and Hunter being the notable X-linked exceptions.

Question 48: An infant presents with an eczematous rash, rhythmic rocking, microcephaly, and hypotonia. What is the likely diagnosis?

A. Phenylketonuria (Correct Answer)
B. Homocystinuria
C. Tyrosinosis
D. Autism

Explanation: **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase (PAH)** or its cofactor **tetrahydrobiopterin (BH4)**. This leads to the accumulation of phenylalanine and its metabolites (phenylpyruvate, phenyllactate) in the blood and brain. **Why Option A is correct:** The clinical presentation described is classic for untreated PKU: * **Eczematous rash:** A common dermatological manifestation due to melanin deficiency (phenylalanine inhibits tyrosinase). * **Neurological symptoms:** Rhythmic rocking, microcephaly, and hypotonia are signs of intellectual disability and CNS involvement caused by the neurotoxic effects of high phenylalanine levels. * **Mousy/Musty Odor:** (Though not mentioned here) is a hallmark sign due to phenylacetic acid in sweat and urine. **Why incorrect options are wrong:** * **Homocystinuria:** Characterized by a "marfanoid habitus," ectopia lentis (downward dislocation), and thromboembolic episodes. It does not typically present with an eczematous rash. * **Tyrosinosis (Tyrosinemia Type I):** Presents primarily with liver failure (cabbage-like odor), renal tubular dysfunction (Fanconi syndrome), and rickets. * **Autism:** While rhythmic rocking is a behavioral feature of autism, it does not explain the microcephaly, hypotonia, or the eczematous rash in an infant. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Defect:** Most commonly Phenylalanine Hydroxylase (PAH). * **Dietary Management:** Low phenylalanine diet; Tyrosine becomes an **essential amino acid** for these patients. * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Maternal PKU:** If a mother has high Phe levels during pregnancy, the fetus may develop microcephaly, CHD, and IUGR, even if the fetus is heterozygous.

Question 49: Which chromosome is defective in cystic fibrosis?

A. Chromosome 5
B. Chromosome X
C. Chromosome 7 (Correct Answer)
D. Chromosome 1

Explanation: **Explanation:** **Cystic Fibrosis (CF)** is an autosomal recessive multisystem disorder caused by a mutation in the **CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene**. This gene is located on the **long arm (q) of Chromosome 7** (specifically 7q31.2). The CFTR protein functions as a cAMP-regulated chloride channel; its defect leads to thick, viscous secretions in the lungs, pancreas, and reproductive tract. **Analysis of Options:** * **Chromosome 7 (Correct):** Houses the CFTR gene. The most common mutation is **ΔF508** (deletion of phenylalanine at position 508), which leads to protein misfolding and degradation in the endoplasmic reticulum. * **Chromosome 5:** Associated with conditions like **Cri-du-chat syndrome** (5p deletion) and **Familial Adenomatous Polyposis (FAP)** (APC gene). * **Chromosome X:** Associated with X-linked disorders such as **Duchenne Muscular Dystrophy (DMD)**, Hemophilia, and G6PD deficiency. * **Chromosome 1:** Associated with conditions like **Gaucher disease** (GBA gene) and Factor V Leiden deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** The gold standard is the **Pilocarpine Iontophoresis (Sweat Chloride Test)**; chloride levels >60 mEq/L are diagnostic. * **Infertility:** 95% of males with CF have **Congenital Bilateral Absence of the Vas Deferens (CBAVD)**. * **Microbiology:** *Pseudomonas aeruginosa* is the most common cause of recurrent pulmonary infections in older CF patients. * **GI Manifestation:** Meconium ileus in newborns is a classic early presentation.

Question 50: Congenital lactic acidosis may occur due to a defect in which of the following?

A. Pyruvate decarboxylase
B. Pyruvate dehydrogenase complex (Correct Answer)
C. Transketolase
D. Alpha-ketoglutarate dehydrogenase

Explanation: **Explanation:** **1. Why Pyruvate Dehydrogenase (PDH) Complex is Correct:** The PDH complex is a multi-enzyme cluster that converts Pyruvate into Acetyl-CoA, serving as the critical bridge between glycolysis and the TCA cycle. When this complex is deficient, pyruvate cannot enter the mitochondria for aerobic metabolism. Instead, it is shunted toward the alternative anaerobic pathway, where it is converted into **Lactic Acid** by the enzyme Lactate Dehydrogenase (LDH). This leads to a buildup of lactate in the blood and CSF, manifesting as **Congenital Lactic Acidosis**. **2. Why Incorrect Options are Wrong:** * **Pyruvate Decarboxylase (A):** This enzyme is part of the fermentation process in yeast and bacteria (converting pyruvate to acetaldehyde); it is not present in humans. * **Transketolase (C):** This is an enzyme of the Pentose Phosphate Pathway (HMP Shunt). While it requires Thiamine (B1) as a cofactor, its deficiency does not cause lactic acidosis; it is typically used to clinically diagnose Thiamine deficiency. * **Alpha-ketoglutarate Dehydrogenase (D):** While this enzyme is structurally similar to PDH and also requires Thiamine, its deficiency is rare and typically presents with neurological impairment rather than primary congenital lactic acidosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** PDH deficiency is the most common enzymatic cause of congenital lactic acidosis and is most frequently inherited as an **X-linked dominant** trait (E1-alpha subunit mutation). * **Clinical Presentation:** Severe cases present in infancy with psychomotor retardation, hypotonia, and seizures. * **Dietary Management:** Patients are managed with a **Ketogenic Diet** (high fat, low carbohydrate). This provides an alternative fuel source (ketone bodies) that bypasses the PDH block to produce Acetyl-CoA. * **Glucogenic vs. Ketogenic:** In these patients, purely ketogenic amino acids (**Leucine and Lysine**) are essential as they do not increase lactate levels.

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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