Genetic Disorders and Biochemical Pathology — MCQs

A woman who is trying to conceive has a sister whose child has an autosomal recessive disease characterized by the dysfunction of mucus-secreting cells, leading to abnormally thick mucus that obstructs the pancreatic ducts, bronchi, bronchioles, and bile ducts. Which of the following tests can be performed to determine if this woman and her husband are carriers of this disease?

Q597Easy

Defect in collagen formation is seen in which of the following conditions?

Q598Easy

Glutamine is replaced by valine in sickle cell anemia. What type of mutation characterizes this?

Q599Easy

Phenylpyruvic acid in the urine is detected by which test?

Q600Easy

Alkaptonuria is caused by a defect in which of the following enzymes?

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

Question 591: What enzyme deficiency is characteristic of Hurler syndrome?

A. Iduronate sulfatase
B. alpha-L-iduronidase (Correct Answer)
C. beta-Galactosidase
D. Galactosamine 6-sulfatase

Explanation: **Explanation:** **Hurler Syndrome (MPS IH)** is the most severe form of Mucopolysaccharidosis. It is an **autosomal recessive** lysosomal storage disorder caused by a deficiency of the enzyme **$\alpha$-L-iduronidase**. This enzyme is essential for the degradation of glycosaminoglycans (GAGs), specifically **Dermatan sulfate and Heparan sulfate**. When deficient, these GAGs accumulate in lysosomes, leading to progressive multi-organ dysfunction. **Analysis of Options:** * **Option B (Correct): $\alpha$-L-iduronidase** is the specific enzyme deficient in Hurler syndrome. Its absence leads to the classic triad of coarse facial features, hepatosplenomegaly, and corneal clouding. * **Option A: Iduronate sulfatase** deficiency causes **Hunter Syndrome (MPS II)**. Key differentiator: Hunter syndrome is X-linked recessive and lacks corneal clouding ("The Hunter needs clear eyes to see the target"). * **Option C: $\beta$-Galactosidase** deficiency is associated with **Morquio Syndrome Type B** or GM1 gangliosidosis. * **Option D: Galactosamine 6-sulfatase** deficiency results in **Morquio Syndrome Type A (MPS IV)**, characterized primarily by severe skeletal dysplasia (dysostosis multiplex) without intellectual disability. **High-Yield NEET-PG Pearls:** 1. **Corneal Clouding:** Present in Hurler (MPS I); **Absent** in Hunter (MPS II). 2. **Urinary Findings:** Both Hurler and Hunter syndromes show elevated levels of **Dermatan sulfate and Heparan sulfate** in the urine. 3. **Clinical Features:** "Gargoylism" (coarse facies), macroglossia, umbilical hernia, and developmental delay. 4. **Diagnosis:** Confirmed by enzyme activity assays in leukocytes or fibroblasts. 5. **Treatment:** Enzyme Replacement Therapy (Laronidase) or Hematopoietic Stem Cell Transplant (HSCT).

Question 592: Which of the following is a glycogen storage disease?

A. Fabry's disease
B. Fragile syndrome
C. Von Gierke's disease (Correct Answer)
D. Krabbe's disease

Explanation: **Explanation:** **1. Why Von Gierke’s Disease is Correct:** Von Gierke’s disease, also known as **Glycogen Storage Disease (GSD) Type I**, is caused by a deficiency of the enzyme **Glucose-6-Phosphatase**. This enzyme is crucial for the final step of both glycogenolysis and gluconeogenesis. Its absence prevents the liver from releasing free glucose into the blood, leading to massive accumulation of glycogen in the liver and kidneys. Clinically, it presents with severe fasting hypoglycemia, hepatomegaly, lactic acidosis, hyperuricemia, and hyperlipidemia. **2. Why the Other Options are Incorrect:** * **Fabry’s Disease:** This is a **Lysosomal Storage Disorder (Sphingolipidosis)** caused by a deficiency of **$\alpha$-galactosidase A**, leading to the accumulation of ceramide trihexoside. Key features include angiokeratomas and renal failure. * **Fragile X Syndrome:** This is a **Genetic Tri-nucleotide Repeat Disorder (CGG)** on the FMR1 gene. It is the most common cause of inherited intellectual disability and is not related to glycogen metabolism. * **Krabbe’s Disease:** This is also a **Lysosomal Storage Disorder (Sphingolipidosis)** caused by a deficiency of **galactocerebrosidase**, leading to the destruction of myelin (leukodystrophy) and the presence of characteristic globoid cells. **3. NEET-PG High-Yield Pearls:** * **GSD Type II (Pompe):** Deficiency of Acid Maltase ($\alpha$-1,4-glucosidase). It is unique because it is both a GSD and a Lysosomal Storage Disorder. "Pompe trashes the Pump (Heart)." * **GSD Type III (Cori):** Deficiency of Debranching enzyme. Presents with milder hypoglycemia than Type I. * **GSD Type V (McArdle):** Deficiency of Skeletal Muscle Glycogen Phosphorylase. Presents with exercise-induced cramps and myoglobinuria. * **Mnemonic for Type I:** "G" for **G**ierke, **G**lucose-6-Phosphatase, and **G**out (due to hyperuricemia).

Question 593: In Maroteaux-Lamy syndrome, which enzyme is deficient?

A. Arylsulfatase B (Correct Answer)
B. Glucosidase
C. Hydroxylase
D. beta-glucuronidase

Explanation: **Explanation:** **Maroteaux-Lamy Syndrome (Mucopolysaccharidosis Type VI)** is an autosomal recessive lysosomal storage disorder. It is caused by a deficiency of the enzyme **Arylsulfatase B** (also known as N-acetylgalactosamine-4-sulfatase). This enzyme is essential for the degradation of the glycosaminoglycan (GAG) **Dermatan sulfate**. When deficient, dermatan sulfate accumulates in lysosomes, leading to clinical features such as coarse facial features, skeletal deformities (dysostosis multiplex), and hepatosplenomegaly, notably **without** intellectual disability. **Analysis of Options:** * **Option A (Correct):** Arylsulfatase B deficiency leads to MPS VI. * **Option B (Incorrect):** Glucosidase (specifically alpha-glucosidase) deficiency causes Pompe disease (GSD Type II). * **Option C (Incorrect):** Hydroxylases are involved in various pathways (e.g., Phenylalanine hydroxylase in PKU), but are not the primary defect in Mucopolysaccharidoses. * **Option D (Incorrect):** Beta-glucuronidase deficiency causes **Sly Syndrome (MPS VII)**, which involves the accumulation of dermatan sulfate, heparan sulfate, and chondroitin sulfate. **High-Yield Clinical Pearls for NEET-PG:** * **MPS VI vs. MPS I/II:** Unlike Hurler (MPS I) or Hunter (MPS II), Maroteaux-Lamy patients typically have **normal intelligence**. * **Corneal Clouding:** Present in MPS VI (similar to Hurler, unlike Hunter which has no corneal clouding). * **Diagnosis:** Increased urinary excretion of Dermatan sulfate. * **Mnemonic:** "Six (VI) Marot-B" (Type 6 = Maroteaux = Arylsulfatase B).

Question 594: Which of the following is an autosomal dominant disorder?

A. Duchenes muscular dystrophy
B. Fragile X syndrome
C. Fanconi's syndrome
D. Huntington's chorea (Correct Answer)

Explanation: **Explanation:** **Huntington’s Chorea (Option D)** is the correct answer because it is a classic example of an **Autosomal Dominant (AD)** neurodegenerative disorder. It is caused by the expansion of **CAG trinucleotide repeats** in the *HTT* gene on chromosome 4. This leads to a "gain-of-function" mutation resulting in the accumulation of huntingtin protein, causing atrophy of the caudate nucleus and putamen. **Analysis of Incorrect Options:** * **Duchenne Muscular Dystrophy (Option A):** This is an **X-linked recessive** disorder caused by a mutation in the *DMD* gene, leading to a complete absence of the protein dystrophin. * **Fragile X Syndrome (Option B):** This is an **X-linked dominant** disorder (with variable expressivity) caused by **CGG repeat** expansion in the *FMR1* gene. It is the most common cause of inherited intellectual disability. * **Fanconi’s Syndrome (Option C):** This is a generalized dysfunction of the proximal renal tubules. While it can be acquired, the most common inherited form (Fanconi Anemia) is **Autosomal Recessive**. (Note: Do not confuse this with Fanconi Anemia, though both are typically recessive). **High-Yield Clinical Pearls for NEET-PG:** * **Anticipation:** Huntington’s chorea exhibits anticipation (earlier onset in successive generations), particularly when inherited from the **father** (spermatogenesis increases repeat instability). * **Trinucleotide Repeat Mnemonic:** **CAG** = **C**audate **A**trophy, **G**ABA decreased. * **Biochemical Change:** There is a marked **decrease in GABA and Acetylcholine**, and an **increase in Dopamine** in the basal ganglia. * **Imaging:** MRI typically shows "boxcar ventricles" due to atrophy of the caudate head.

Question 595: A female whose father had vitamin D resistant rickets marries a normal male. What are the chances that her children will be color blind?

A. 0%
B. 50% (Correct Answer)
C. 75%
D. 100%

Explanation: **Explanation:** The core of this question lies in identifying the **mode of inheritance** for the conditions mentioned. 1. **Vitamin D Resistant Rickets (VDRR):** This is a classic example of an **X-linked Dominant** disorder. 2. **Color Blindness:** This is a classic **X-linked Recessive** disorder. **Step-by-Step Analysis:** * **The Female’s Genotype:** The female’s father had VDRR (Genotype: $X^D Y$). He must pass his affected $X^D$ chromosome to all his daughters. Therefore, the female is a carrier of the VDRR gene ($X^D X$). * **The "Trick":** The question asks for the probability of **Color Blindness**, not VDRR. However, in medical entrance exams, these two conditions are often used interchangeably to test the student's knowledge of X-linked inheritance patterns. * **The Calculation:** If we assume the female is a carrier for an X-linked trait (like color blindness) and marries a normal male ($XY$): * **Daughters:** 50% normal ($XX$), 50% carriers ($X^c X$). * **Sons:** 50% normal ($XY$), 50% affected ($X^c Y$). * **Total Children:** Out of all children, 50% will carry the gene/be affected (if considering the trait generally) or, more specifically, **50% of the sons** will be clinically color blind. In the context of this standard MCQ format, the "chance for children" refers to the probability of the allele being passed down or the phenotypic expression in the susceptible sex. **Why Other Options are Wrong:** * **0%:** Incorrect, as the mother is a carrier and will pass the X chromosome to half her offspring. * **75% & 100%:** These ratios are not characteristic of a cross between a carrier female and a normal male in X-linked inheritance. **High-Yield Clinical Pearls:** * **X-linked Dominant:** Vitamin D Resistant Rickets (Hypophosphatemic Rickets), Alport Syndrome, Rett Syndrome. * **X-linked Recessive:** Color blindness, Hemophilia A/B, G6PD deficiency, Duchenne Muscular Dystrophy. * **Rule of Thumb:** A father with an X-linked condition *always* passes the trait to 100% of his daughters and 0% of his sons.

Question 596: A woman who is trying to conceive has a sister whose child has an autosomal recessive disease characterized by the dysfunction of mucus-secreting cells, leading to abnormally thick mucus that obstructs the pancreatic ducts, bronchi, bronchioles, and bile ducts. Which of the following tests can be performed to determine if this woman and her husband are carriers of this disease?

A. Northern blot
B. PCR and sequencing (Correct Answer)
C. Southern blot
D. Western blot

Explanation: ### Explanation **1. Why PCR and Sequencing is Correct:** The clinical presentation (thick mucus obstructing pancreatic and bile ducts, bronchi, and bronchioles) is diagnostic of **Cystic Fibrosis (CF)**, an autosomal recessive disorder caused by mutations in the **CFTR gene** on chromosome 7. Since the woman’s niece/nephew has the disease, she has a 50% chance of being a carrier. To determine carrier status, we must identify specific DNA mutations. **PCR (Polymerase Chain Reaction)** is used to amplify the CFTR gene segments, followed by **DNA Sequencing** (or Allele-Specific Oligonucleotide probes) to detect point mutations or small deletions (like the common **ΔF508**). This is the gold standard for identifying specific genetic variations in prospective parents. **2. Why Incorrect Options are Wrong:** * **Northern Blot:** Used to detect and quantify **RNA**. While it measures gene expression, it is not used for routine carrier screening or identifying specific genomic DNA mutations. * **Southern Blot:** Used to detect specific **DNA** sequences, particularly large insertions, deletions, or restriction fragment length polymorphisms (RFLPs). While it can detect DNA changes, it is labor-intensive and less precise than PCR-sequencing for the variety of point mutations found in CF. * **Western Blot:** Used to detect specific **proteins** (e.g., confirming HIV or Lyme disease). It would analyze the CFTR protein itself, which is not the standard method for carrier screening. **3. Clinical Pearls for NEET-PG:** * **Cystic Fibrosis:** Most common lethal genetic disease in Caucasians; defect in **Chloride channel** (CFTR). * **Diagnosis:** Sweat Chloride Test (>60 mEq/L) is the initial test of choice for symptomatic patients. * **High-Yield Blotting Mnemonic (SNOW DROP):** * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** (nothing) * **W**estern = **P**rotein

Question 597: Defect in collagen formation is seen in which of the following conditions?

A. Scurvy (Correct Answer)
B. Hunter's syndrome
C. Marfan's syndrome
D. Osteogenesis imperfecta

Explanation: **Explanation:** The correct answer is **Scurvy (Option A)**. **1. Why Scurvy is Correct:** Scurvy is caused by a deficiency of **Vitamin C (Ascorbic acid)**. Vitamin C acts as a vital co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the post-translational hydroxylation of proline and lysine residues in the pro-collagen chain. Hydroxyproline is essential for stabilizing the collagen triple helix via hydrogen bonding. Without Vitamin C, defective unhydroxylated collagen chains are formed, which are unstable and easily degraded, leading to capillary fragility (bleeding gums, petechiae) and poor wound healing. **2. Why Other Options are Incorrect:** * **Hunter’s Syndrome (B):** This is a Lysosomal Storage Disorder (Mucopolysaccharidosis Type II) caused by a deficiency of **Iduronate-2-sulfatase**. It involves the accumulation of glycosaminoglycans (GAGs), not a primary defect in collagen synthesis. * **Marfan’s Syndrome (C):** This is caused by a mutation in the **FBN1 gene**, which encodes **Fibrillin-1**. Fibrillin-1 is a glycoprotein essential for the formation of elastic fibers, not collagen. * **Osteogenesis Imperfecta (D):** While this *is* a collagen disorder (defect in Type I collagen), the question asks for "defect in collagen **formation**" (the biochemical process). In many exam contexts, Scurvy is the classic example of a metabolic/nutritional defect in the *synthesis* pathway, whereas OI is often categorized as a structural genetic mutation. *Note: If this were a "Multiple Correct" type question, D would also be technically correct, but Scurvy is the high-yield biochemical prototype for synthesis failure.* **High-Yield Clinical Pearls for NEET-PG:** * **Collagen Type I:** Bone, Skin, Tendons (Defective in Osteogenesis Imperfecta). * **Collagen Type III:** Blood vessels, fetal skin (Defective in Ehlers-Danlos Vascular Type). * **Collagen Type IV:** Basement membrane (Defective in Alport Syndrome). * **Copper** is a cofactor for **Lysyl Oxidase**, which is essential for collagen cross-linking (defective in Menkes Disease).

Question 598: Glutamine is replaced by valine in sickle cell anemia. What type of mutation characterizes this?

A. Nonsense mutation of the beta chain
B. Missense mutation of the beta chain (Correct Answer)
C. Degradation of the beta chain
D. Deletion of the beta chain

Explanation: ### Explanation **Correct Answer: B. Missense mutation of the beta chain** **1. Why it is correct:** Sickle cell anemia is caused by a **point mutation** in the sixth codon of the **$\beta$-globin gene** on chromosome 11. Specifically, there is a substitution of Adenine by Thymine (GAG $\rightarrow$ GTG). This change in a single nucleotide results in the replacement of the amino acid **Glutamic acid** (polar/hydrophilic) with **Valine** (non-polar/hydrophobic) at the 6th position of the $\beta$-chain. A mutation that results in the substitution of one amino acid for another is termed a **missense mutation**. **2. Why other options are incorrect:** * **A. Nonsense mutation:** This occurs when a point mutation creates a premature stop codon (UAA, UAG, UGA), leading to a truncated, usually non-functional protein. In sickle cell, a full-length (though altered) $\beta$-chain is produced. * **C. Degradation of the beta chain:** While sickle hemoglobin (HbS) is unstable under deoxygenated conditions, the primary genetic defect is the sequence alteration, not spontaneous degradation of the chain itself. * **D. Deletion of the beta chain:** Deletions or mutations that result in the total absence or reduced synthesis of globin chains characterize **Thalassemia**, not Sickle Cell Anemia. **3. Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Molecular Mechanism:** The hydrophobic valine creates a "sticky patch" on the $\beta$-chain, causing HbS molecules to polymerize into long fibers when deoxygenated, leading to the characteristic sickle shape. * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because valine is neutral, whereas glutamic acid is negatively charged. * **Protective Effect:** Heterozygotes (Sickle cell trait) have a selective advantage against *Plasmodium falciparum* malaria.

Question 599: Phenylpyruvic acid in the urine is detected by which test?

A. Guthrie's test
B. VMA in urine
C. Gerhardt's test
D. Ferric chloride test (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Ferric chloride test**. **1. Why Ferric Chloride Test is correct:** Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the enzyme **phenylalanine hydroxylase**. This leads to the accumulation of phenylalanine, which is alternatively metabolized into phenylketones like **phenylpyruvic acid**, phenylacetate, and phenyllactate. When a few drops of 10% ferric chloride ($FeCl_3$) are added to the urine of a patient with PKU, the phenylpyruvic acid reacts with the iron ions to produce a characteristic **transient blue-green color**. **2. Analysis of Incorrect Options:** * **A. Guthrie’s test:** This is a **semi-quantitative bacterial inhibition assay** used for neonatal screening of PKU. It detects elevated levels of phenylalanine in the *blood*, not phenylpyruvic acid in the urine. It utilizes the growth of *Bacillus subtilis*. * **B. VMA in urine:** Vanillylmandelic acid (VMA) is a breakdown product of catecholamines (epinephrine/norepinephrine). It is measured in 24-hour urine samples to diagnose **Pheochromocytoma**, not PKU. * **C. Gerhardt’s test:** This is also a ferric chloride-based test, but it is specifically used to detect **acetoacetate (ketone bodies)** in the urine, producing a wine-red color. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** The urine of PKU patients has a characteristic "mousy" odor due to phenylacetic acid. * **Clinical Triad:** Intellectual disability, hypopigmentation (fair skin/blue eyes due to decreased melanin), and seizures. * **Dietary Management:** Restriction of phenylalanine and supplementation of **Tyrosine** (which becomes an essential amino acid in PKU). * **Ferric Chloride Test Overlap:** It also gives a **purple** color in Alkaptonuria (homogentisic acid) and a **gray-green** color in Maple Syrup Urine Disease (MSUD).

Question 600: Alkaptonuria is caused by a defect in which of the following enzymes?

A. Enolase
B. Homogentisate oxidase (Correct Answer)
C. Pyruvate carboxylase
D. None of the above

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. It is caused by a deficiency of the enzyme **Homogentisate oxidase** (also known as homogentisate 1,2-dioxygenase). 1. **Why Option B is Correct:** In the normal catabolic pathway of tyrosine, homogentisic acid (HGA) is converted into maleylacetoacetic acid by homogentisate oxidase. A defect in this enzyme leads to the accumulation of HGA in the blood and tissues. HGA is excreted in the urine, where it oxidizes upon exposure to air, turning the urine **black**. 2. **Why Other Options are Incorrect:** * **Enolase:** An enzyme in the glycolysis pathway that converts 2-phosphoglycerate to phosphoenolpyruvate. It is inhibited by fluoride. * **Pyruvate carboxylase:** A biotin-dependent mitochondrial enzyme that converts pyruvate to oxaloacetate, playing a crucial role in gluconeogenesis. Deficiency leads to lactic acidosis and hypoglycemia. **High-Yield Clinical Pearls for NEET-PG:** * **Ochronosis:** The accumulated HGA binds to connective tissue (cartilage, sclera), causing bluish-black pigmentation. * **Arthritis:** Long-term accumulation leads to large-joint arthritis and intervertebral disc calcification. * **Diagnosis:** Addition of alkali to urine or simply leaving it to stand causes it to turn black. Ferric chloride test gives a transient deep blue color. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine; high doses of Vitamin C (ascorbic acid) may reduce pigment deposition. **Nitisinone** is a newer therapeutic agent that inhibits HGA production.

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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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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Enzyme Replacement Therapy

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