Genetic Disorders and Biochemical Pathology — MCQs

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

Question 321: Which substance is typically excreted in lead poisoning?

A. Urobilinogen
B. Coproporphyrin (Correct Answer)
C. Bilirubin
D. Bile salts

Explanation: **Explanation:** Lead poisoning (Plumbism) interferes with the heme biosynthetic pathway by inhibiting two key enzymes: **ALA Dehydratase** and **Ferrochelatase**. 1. **Why Coproporphyrin is correct:** Lead inhibits the enzyme **Coproporphyrinogen oxidase**, which normally converts Coproporphyrinogen III to Protoporphyrin IX. When this enzyme is inhibited, Coproporphyrinogen III accumulates and is spontaneously oxidized into **Coproporphyrin III**, which is then excreted in excess in the urine. This is a classic biochemical marker used for screening lead exposure. 2. **Why the other options are incorrect:** * **Urobilinogen:** This is a byproduct of bilirubin degradation by intestinal bacteria. Increased levels are seen in hemolytic anemias or liver disease, not specifically in lead poisoning. * **Bilirubin:** Elevated levels (jaundice) indicate hemolysis, biliary obstruction, or hepatic dysfunction. While lead can cause hemolytic anemia, bilirubin is not the characteristic substance excreted. * **Bile salts:** These are synthesized in the liver from cholesterol and aid in fat digestion. Their presence in urine (choluria) typically indicates obstructive jaundice. **High-Yield Clinical Pearls for NEET-PG:** * **Enzymes inhibited by Lead:** ALA Dehydratase (most sensitive) and Ferrochelatase (leads to elevated Free Erythrocyte Protoporphyrin). * **Diagnostic Marker:** Elevated **Urinary Delta-Aminolevulinic Acid (δ-ALA)** is also a hallmark of lead poisoning. * **Hematological finding:** **Basophilic stippling** of RBCs (due to inhibition of pyrimidine 5'-nucleotidase). * **Clinical Signs:** Burton’s lines (blue gums), wrist drop/foot drop, and abdominal colic.

Question 322: Which genetic disease is exclusively transmitted to females?

A. Hurler's disease
B. Hunter's disease (Correct Answer)
C. Scheie's disease
D. Fabry's disease

Explanation: ### Explanation **Correct Answer: B. Hunter’s disease** The question focuses on the mode of inheritance of Mucopolysaccharidoses (MPS). * **Hunter’s disease (MPS II)** is unique among the mucopolysaccharidoses because it is inherited in an **X-linked recessive** pattern. * In X-linked recessive disorders, a carrier mother has a 50% chance of transmitting the defective gene to her offspring. While males are primarily affected (as they are hemizygous), **females can be carriers** and, in rare cases of skewed X-inactivation (Lyonization), can manifest the disease. * *Note on Question Phrasing:* In medical entrance exams, "transmitted to females" often refers to the carrier status or the genetic transmission through the maternal line, distinguishing it from Autosomal Recessive traits where both parents must contribute a gene. **Why the other options are incorrect:** * **A & C (Hurler’s and Scheie’s disease):** Both are subtypes of **MPS I** (deficiency of α-L-iduronidase). All forms of MPS I are inherited in an **Autosomal Recessive** manner, affecting males and females equally. * **D (Fabry’s disease):** While Fabry’s is also X-linked recessive, it is a **Sphingolipidosis**, not a Mucopolysaccharidosis. In the context of comparing MPS types (the primary theme of the options), Hunter’s is the classic "exception to the rule." **High-Yield Clinical Pearls for NEET-PG:** 1. **Mnemonic:** "The **Hunter** needs **X**-ray vision to see the **Target** (No corneal clouding)." * **X:** X-linked recessive inheritance. * **Target:** Presence of "Target-like" skin lesions (Pebbling). 2. **Key Distinction:** Unlike Hurler’s (MPS I), Hunter’s (MPS II) presents **without corneal clouding**. 3. **Enzyme Deficiency:** Hunter’s is caused by a deficiency of **Iduronate-2-sulfatase**. 4. **Accumulated Substances:** Dermatan sulfate and Heparan sulfate.

Question 323: Mitochondrial DNA linked disease is characterized by which mode of inheritance?

A. More common in males
B. Transmitted by females (Correct Answer)
C. Variable penetrance in families
D. Autosomal inheritance

Explanation: **Explanation:** **Mitochondrial inheritance** (also known as maternal inheritance) is a non-Mendelian pattern of inheritance governed by the DNA found within mitochondria (mtDNA). **1. Why "Transmitted by females" is correct:** During fertilization, the zygote receives almost all its cytoplasm and organelles from the **oocyte**. The sperm contributes only its nuclear DNA; its mitochondria are located in the tail, which either does not enter the egg or is selectively degraded post-fertilization. Therefore, a mother will pass the mitochondrial trait to **all** her children (both sons and daughters), but only the daughters can pass it to the next generation. **2. Analysis of Incorrect Options:** * **A. More common in males:** This describes X-linked recessive disorders (e.g., Hemophilia). Mitochondrial diseases affect males and females equally. * **C. Variable penetrance in families:** While mitochondrial diseases show **variable expressivity** due to *heteroplasmy* (a mix of normal and mutated mtDNA), "variable penetrance" is more characteristic of autosomal dominant conditions. In mitochondrial inheritance, the hallmark is the specific maternal transmission pattern. * **D. Autosomal inheritance:** This refers to genes located on the 22 pairs of non-sex chromosomes (nuclear DNA), which follow Mendelian laws. **High-Yield Clinical Pearls for NEET-PG:** * **Heteroplasmy:** The coexistence of mutated and wild-type mtDNA in a single cell. The severity of the disease depends on the ratio of mutated to normal mitochondria. * **Threshold Effect:** Symptoms appear only when the proportion of mutated mtDNA exceeds a specific level. * **Tissues Affected:** Organs with high energy demands are most affected (CNS, Skeletal muscle, Heart). * **Classic Examples:** LHON (Leber’s Hereditary Optic Neuropathy), MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes), and MERRF (Myoclonic Epilepsy with Ragged Red Fibers).

Question 324: Which of the following conditions is NOT associated with a defect in DNA repair mechanisms?

A. Xeroderma Pigmentosa
B. Fanconi anemia
C. Huntington's disease (Correct Answer)
D. Ataxia-telangiectasia

Explanation: **Explanation:** The correct answer is **Huntington’s disease** because it is a **trinucleotide repeat expansion disorder** (CAG repeats on chromosome 4), not a primary defect in DNA repair. It is characterized by the toxic gain-of-function of the huntingtin protein, leading to neurodegeneration in the striatum (caudate nucleus). **Analysis of Options:** * **Xeroderma Pigmentosum (XP):** A classic DNA repair defect caused by a deficiency in **Nucleotide Excision Repair (NER)**. Patients cannot repair pyrimidine dimers formed by UV light, leading to extreme photosensitivity and early-onset skin cancers. * **Fanconi Anemia:** Caused by defects in a cluster of proteins responsible for repairing **DNA interstrand cross-links**. It presents with bone marrow failure, physical anomalies (e.g., absent radii/thumbs), and a high risk of AML. * **Ataxia-telangiectasia:** Caused by a mutation in the **ATM gene**, which is essential for detecting **DNA double-strand breaks** and activating the p53 pathway. It presents with cerebellar ataxia, telangiectasias, and immunodeficiency. **Clinical Pearls for NEET-PG:** 1. **DNA Repair Mnemonic:** * **NER** defect $\rightarrow$ Xeroderma Pigmentosum. * **Mismatch Repair (MMR)** defect $\rightarrow$ Lynch Syndrome (HNPCC). * **Double-strand break** defect $\rightarrow$ Ataxia-telangiectasia & BRCA1/2 mutations. 2. **Huntington’s Disease** shows **Anticipation** (earlier onset in successive generations), typically during paternal transmission. 3. **Diagnostic Test for Fanconi Anemia:** Chromosomal breakage study using Diepoxybutane (DEB) or Mitomycin C.

Question 325: G-6-Phosphatase deficiency is seen in which of the following conditions?

A. Von Gierke's disease (Correct Answer)
B. Tay-Sachs disease
C. Pompe's disease
D. Anderson's disease

Explanation: **Explanation:** **Von Gierke’s Disease (Type I Glycogen Storage Disease)** is the correct answer. It is caused by a deficiency of the enzyme **Glucose-6-Phosphatase**, which is responsible for the final step of both glycogenolysis and gluconeogenesis (converting Glucose-6-Phosphate to free Glucose). Since this enzyme is primarily located in the liver and kidneys, its deficiency leads to severe fasting hypoglycemia and the accumulation of glycogen in these organs, resulting in hepatomegaly. **Analysis of Incorrect Options:** * **Tay-Sachs Disease:** This is a lysosomal storage disorder (Sphingolipidosis) caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides. It is characterized by neurodegeneration and a cherry-red spot on the macula. * **Pompe’s Disease (Type II GSD):** This is caused by a deficiency of **Lysosomal acid alpha-1,4-glucosidase** (Acid Maltase). Unlike Type I, it affects the heart and muscles, leading to massive cardiomegaly. * **Anderson’s Disease (Type IV GSD):** This results from a deficiency of the **Branching enzyme**. It leads to the accumulation of abnormal glycogen with long outer branches (amylopectin-like), causing early-onset liver cirrhosis. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Triad of Von Gierke’s:** Hyperuricemia (leading to gout), Hyperlipidemia, and Lactic Acidosis. * **Diagnostic Hallmark:** Failure of blood glucose levels to rise after the administration of glucagon or epinephrine. * **Type Ia vs. Ib:** Type Ia is a deficiency of the enzyme itself, while Type Ib is a deficiency of the **Glucose-6-Phosphate translocase** (associated with neutropenia).

Question 326: A one-month-old male infant presents with feeding difficulties and a history of frequent seizures. Blood investigations reveal elevated very long chain fatty acids (VLCFA). What is the likely diagnosis?

A. Refsum disease
B. Zellweger syndrome
C. Cerebrohepatorenal syndrome
D. Zellweger syndrome and Cerebrohepatorenal syndrome (Correct Answer)

Explanation: **Explanation:** The correct answer is **D (Zellweger syndrome and Cerebrohepatorenal syndrome)** because they are synonymous terms for the same clinical entity. **1. Understanding the Diagnosis:** Zellweger syndrome (also known as Cerebrohepatorenal syndrome) is the most severe form of **Peroxisome Biogenesis Disorders (PBD)**. It is caused by mutations in *PEX* genes, which are essential for the normal assembly of peroxisomes. Peroxisomes are responsible for the **alpha-oxidation** of branched-chain fatty acids and the **beta-oxidation** of **Very Long Chain Fatty Acids (VLCFA)** (carbon chains >22). In the absence of functional peroxisomes, VLCFAs accumulate in the blood and tissues, particularly the brain and liver, leading to the clinical triad of neurological impairment (seizures, hypotonia), hepatic dysfunction, and renal cysts. **2. Analysis of Options:** * **Option A (Refsum disease):** This is a defect specifically in **alpha-oxidation** due to a deficiency of the enzyme *Phytanoyl-CoA hydroxylase*. While it involves peroxisomal dysfunction, it results in the accumulation of **Phytanic acid**, not VLCFAs. It typically presents later in life with retinitis pigmentosa and ataxia. * **Option B & C:** While both are technically correct, they are incomplete. Since Zellweger syndrome and Cerebrohepatorenal syndrome refer to the same condition, Option D is the most accurate choice for a competitive exam. **3. High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Marker:** Elevated plasma levels of **VLCFA** (C24:0 and C26:0) is the pathognomonic finding. * **Clinical Triad:** Hypotonia ("floppy infant"), seizures, and dysmorphic facial features (high forehead, widened fontanelles). * **Radiological Sign:** Stippled epiphyses (chondrodysplasia punctata) may be seen on X-ray. * **Prognosis:** Usually fatal within the first year of life.

Question 327: Hexosaminidase A deficiency causes which of the following genetic disorders?

A. Tay-Sachs disease (Correct Answer)
B. Niemann-Pick disease
C. Gaucher's disease
D. Krabbe's disease

Explanation: ### Explanation **Correct Answer: A. Tay-Sachs disease** **1. Why Tay-Sachs Disease is Correct:** Tay-Sachs disease is an autosomal recessive lysosomal storage disorder caused by a deficiency of the enzyme **Hexosaminidase A**. This deficiency leads to the toxic accumulation of **GM2 gangliosides**, particularly in the neurons of the brain and spinal cord. Clinically, this manifests as progressive neurodegeneration, developmental delay, and the characteristic **"cherry-red spot"** on the macula (without hepatosplenomegaly). **2. Why the Other Options are Incorrect:** * **B. Niemann-Pick disease:** Caused by a deficiency of **Sphingomyelinase**, leading to the accumulation of sphingomyelin. While it also features a cherry-red spot, it is distinguished from Tay-Sachs by the presence of **hepatosplenomegaly** and "foam cells" on histology. * **C. Gaucher's disease:** The most common lysosomal storage disease, caused by a deficiency of **Glucocerebrosidase** (β-glucosidase). It is characterized by hepatosplenomegaly, bone crises, and "Gaucher cells" (crumpled tissue paper appearance). * **D. Krabbe's disease:** Caused by a deficiency of **Galactocerebrosidase**, leading to the accumulation of galactocerebroside and psychosine. It is characterized by the presence of **globoid cells** and destruction of myelin (demyelination). **3. NEET-PG High-Yield Clinical Pearls:** * **Mnemonic for Tay-Sachs:** "A **Gang** of **Six** (**Hex**) **Small** (**No hepatosplenomegaly**) **Jews** (**Ashkenazi descent**)." * **Key differentiator:** Tay-Sachs = No hepatosplenomegaly; Niemann-Pick = Hepatosplenomegaly. * **Enzyme Subunit:** Hexosaminidase A is composed of alpha and beta subunits; a mutation in the **alpha subunit** (Chromosome 15) causes Tay-Sachs. (Mutation in the beta subunit causes Sandhoff disease).

Question 328: A missense gene mutation in the beta-globin chain is characteristic of which condition?

A. Thalassemia
B. Sickle cell anemia (Correct Answer)
C. Hemoglobin Bart's (Hb Barts)
D. Hemoglobin H (HbH)

Explanation: ### Explanation **Correct Answer: B. Sickle cell anemia** **Why it is correct:** Sickle cell anemia is the classic example of a **missense mutation** (a type of point mutation). Specifically, there is a substitution of **Adenine by Thymine (GAG → GTG)** at the 6th codon of the beta-globin gene. This results in the replacement of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic). This single amino acid change causes Hemoglobin S (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Why the other options are incorrect:** * **A. Thalassemia:** Unlike sickle cell anemia (a qualitative defect), Thalassemia is a **quantitative defect**. It is most commonly caused by **splice-site mutations** or nonsense mutations (in $\beta$-thalassemia) and **large gene deletions** (in $\alpha$-thalassemia), leading to reduced or absent synthesis of globin chains. * **C. Hemoglobin Bart's:** This occurs in severe $\alpha$-thalassemia (hydrops fetalis) where all four $\alpha$-globin genes are deleted. It consists of **gamma-chain tetramers ($\gamma_4$)**. * **D. Hemoglobin H:** This occurs when three $\alpha$-globin genes are deleted. It consists of **beta-chain tetramers ($\beta_4$)**. Both Hb Bart's and HbH are results of gene deletions, not missense mutations. **NEET-PG High-Yield Pearls:** * **Mutation Type:** Transversion (Purine A to Pyrimidine T). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because Valine is less negatively charged than Glutamic acid. * **Protective Effect:** Heterozygotes (Sickle cell trait) show resistance to *Plasmodium falciparum* malaria. * **Metabolic Trigger:** Acidosis, dehydration, and hypoxia shift the oxygen dissociation curve to the right, promoting sickling.

Question 329: Which of the following genetic disorders does not have an available enzyme replacement therapy?

A. Gaucher's disease
B. Tay-Sachs disease (Correct Answer)
C. Pompe's disease
D. Hurler syndrome

Explanation: **Explanation:** The correct answer is **Tay-Sachs disease**. The primary challenge in treating Lysosomal Storage Diseases (LSDs) with Enzyme Replacement Therapy (ERT) is the **Blood-Brain Barrier (BBB)**. ERT involves the intravenous administration of recombinant enzymes, which are large polar molecules that cannot cross the BBB to reach the Central Nervous System (CNS). 1. **Why Tay-Sachs is the correct answer:** Tay-Sachs disease is caused by a deficiency of **Hexosaminidase A**, leading to the accumulation of GM2 gangliosides primarily in the neurons. Because the pathology is almost exclusively neurological, systemic ERT is ineffective as it cannot reach the brain. Currently, management is supportive, though gene therapy and substrate reduction therapy are under investigation. 2. **Why other options are incorrect:** * **Gaucher’s Disease (Type 1):** The first LSD for which ERT (**Imiglucerase**) was developed. Since Type 1 lacks CNS involvement, ERT effectively manages hepatosplenomegaly and bone crises. * **Pompe’s Disease:** Caused by Acid Alpha-glucosidase deficiency. ERT (**Alglucosidase alfa**) is available and targets cardiac and skeletal muscle. * **Hurler Syndrome (MPS I):** Treated with ERT (**Laronidase**). While ERT helps systemic symptoms, it does not fix cognitive decline, which is why Hematopoietic Stem Cell Transplant (HSCT) is often preferred for CNS benefits. **High-Yield Clinical Pearls for NEET-PG:** * **Tay-Sachs Hallmark:** Cherry-red spot on the macula + **No hepatosplenomegaly** (distinguishes it from Niemann-Pick). * **Enzyme Mnemonic:** "Tay-Sa**X** lacks He**X**osaminidase." * **ERT Limitation:** Always remember that standard IV ERT is effective for visceral symptoms but generally fails to treat the neurodegenerative components of LSDs.

Question 330: Which enzyme deficiency causes hemolytic anemia?

A. G-6-PD (Correct Answer)
B. Aldolase
C. Isomerase
D. Enolase

Explanation: **Explanation:** **Correct Option: A. G-6-PD (Glucose-6-Phosphate Dehydrogenase)** G-6-PD is the rate-limiting enzyme of the **Hexose Monophosphate (HMP) Shunt**. Its primary role in red blood cells (RBCs) is to produce **NADPH**, which is essential for maintaining a pool of **reduced glutathione**. Reduced glutathione acts as an antioxidant, neutralizing reactive oxygen species (ROS) like hydrogen peroxide. In G-6-PD deficiency, the inability to regenerate NADPH leads to oxidative damage to hemoglobin, causing it to denature and precipitate as **Heinz bodies**. These damaged RBCs are destroyed in the spleen, resulting in episodic hemolytic anemia, often triggered by fava beans, infections, or drugs (e.g., Primaquine, Sulfa drugs). **Incorrect Options:** * **B, C, and D (Aldolase, Isomerase, Enolase):** These are enzymes involved in **Glycolysis** (Embden-Meyerhof pathway). While a deficiency in Pyruvate Kinase (the last step of glycolysis) is a common cause of non-spherocytic hemolytic anemia, deficiencies in Aldolase, Phosphohexose Isomerase, or Enolase are extremely rare and are not the classic or primary associations for hemolytic anemia in standard medical examinations. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** G-6-PD deficiency is an **X-linked recessive** disorder. * **Morphology:** Look for **"Bite cells"** (degmacytes) and **"Blister cells"** on a peripheral smear, which occur when splenic macrophages pluck out Heinz bodies. * **Protection:** G-6-PD deficiency offers a selective advantage against *Plasmodium falciparum* malaria. * **Diagnosis:** The definitive test is the G-6-PD enzyme assay, but it should not be performed during an acute hemolytic episode as young reticulocytes have higher enzyme levels, potentially yielding a false-normal result.

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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