Genetic Disorders and Biochemical Pathology Practice Questions

Q: What enzyme deficiency is characteristic of Hurler syndrome?

alpha-L-iduronidase. **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).

Q: Which of the following is a glycogen storage disease?

Von Gierke's disease. **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).

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

Arylsulfatase B. **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).

Q: Which of the following is an autosomal dominant disorder?

Huntington's chorea. **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.

Q: 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?

PCR and sequencing. ### 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

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

Missense mutation of the beta chain. ### 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.

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

Ferric chloride test. **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).

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

Homogentisate oxidase. **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.

Question 1

What enzyme deficiency is characteristic of Hurler syndrome?

Accepted Answer

alpha-L-iduronidase

Answer

Iduronate sulfatase

Answer

beta-Galactosidase

Answer

Galactosamine 6-sulfatase

Question 2

Which of the following is a glycogen storage disease?

Accepted Answer

Von Gierke's disease

Answer

Fabry's disease

Answer

Fragile syndrome

Answer

Krabbe's disease

Question 3

In Maroteaux-Lamy syndrome, which enzyme is deficient?

Accepted Answer

Arylsulfatase B

Answer

Glucosidase

Answer

Hydroxylase

Answer

beta-glucuronidase

Question 4

Which of the following is an autosomal dominant disorder?

Accepted Answer

Huntington's chorea

Answer

Duchenes muscular dystrophy

Answer

Fragile X syndrome

Answer

Fanconi's syndrome

Question 5

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

Accepted Answer

50%

Answer

0%

Answer

75%

Answer

100%

Question 6

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?

Accepted Answer

PCR and sequencing

Answer

Northern blot

Answer

Southern blot

Answer

Western blot

Question 7

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

Accepted Answer

Scurvy

Answer

Hunter's syndrome

Answer

Marfan's syndrome

Answer

Osteogenesis imperfecta

Question 8

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

Accepted Answer

Missense mutation of the beta chain

Answer

Nonsense mutation of the beta chain

Answer

Degradation of the beta chain

Answer

Deletion of the beta chain

Question 9

Phenylpyruvic acid in the urine is detected by which test?

Accepted Answer

Ferric chloride test

Answer

Guthrie's test

Answer

VMA in urine

Answer

Gerhardt's test

Question 10

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

Accepted Answer

Homogentisate oxidase

Answer

Enolase

Answer

Pyruvate carboxylase

Answer

None of the above

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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