Genetic Disorders and Biochemical Pathology Practice Questions

Q: Metachromatic leukodystrophy is due to deficiency of which enzyme?

Arylsulfatase. **Explanation:** **Metachromatic Leukodystrophy (MLD)** is an autosomal recessive lysosomal storage disorder belonging to the group of sphingolipidoses. 1. **Why Arylsulfatase is correct:** The primary defect in MLD is a deficiency of the enzyme **Arylsulfatase A**. This enzyme is responsible for the degradation of **Sulfatides** (specifically galactosyl sulfatide) into galactocerebroside. When deficient, sulfatides accumulate in the myelin sheaths of both the central and peripheral nervous systems, leading to demyelination. The term "metachromatic" refers to the fact that these accumulated sulfatides change the color of dyes like toluidine blue (shifting from blue to brown/purple) when viewed under a microscope. 2. **Why other options are incorrect:** * **Hexosaminidase A:** Deficiency leads to **Tay-Sachs Disease**, characterized by the accumulation of GM2 gangliosides and a cherry-red spot on the macula (no hepatosplenomegaly). * **Hexosaminidase B:** Deficiency (along with Hexosaminidase A) leads to **Sandhoff Disease**, which presents similarly to Tay-Sachs but involves systemic organ involvement. * **Ceramidase:** Deficiency leads to **Farber Disease**, characterized by painful joint swelling, hoarseness (laryngeal involvement), and subcutaneous nodules. **High-Yield Clinical Pearls for NEET-PG:** * **Accumulated Substrate:** Sulfatide. * **Clinical Presentation:** Progressive motor regression, ataxia, and dementia. * **Diagnosis:** Low Arylsulfatase A activity in leukocytes and presence of metachromatic granules in urine sediment. * **MRI Finding:** "Tigroid pattern" of demyelination in the white matter.

Q: Wilson's disease is caused by a defect in which gene?

ATP7B mutation. **Explanation:** Wilson’s disease (Hepatolenticular degeneration) is an **autosomal recessive** disorder of copper metabolism. The correct answer is **ATP7B mutation** because this gene, located on **Chromosome 13**, encodes a copper-transporting P-type ATPase expressed primarily in the liver. This protein is essential for two processes: the incorporation of copper into apoceruloplasmin to form holoceruloplasmin and the excretion of excess copper into the bile. A defect leads to toxic copper accumulation in the liver, brain (basal ganglia), and cornea. **Analysis of Incorrect Options:** * **ATP7A mutation:** This causes **Menkes Disease** ("Kinky Hair Syndrome"). ATP7A is responsible for intestinal copper absorption; its mutation leads to systemic copper deficiency, unlike the overload seen in Wilson's. * **Ceruloplasmin:** While low serum ceruloplasmin is a diagnostic marker for Wilson’s disease, the primary genetic defect lies in the transporter (ATP7B), not the ceruloplasmin gene itself. * **ATP7C mutation:** This is a distractor; there is no clinically significant human copper disorder currently attributed to an "ATP7C" gene in standard medical curricula. **High-Yield Clinical Pearls for NEET-PG:** * **Kayser-Fleischer (KF) rings:** Copper deposition in the **Descemet’s membrane** of the cornea (best seen on slit-lamp exam). * **Neurological signs:** Wing-beating tremors, dysarthria, and parkinsonian features due to basal ganglia involvement. * **Diagnosis:** Low serum ceruloplasmin, increased urinary copper excretion, and "Giant Panda" sign on MRI brain. * **Treatment:** Chelating agents like **D-Penicillamine** (first-line) or Trientine, and Zinc (inhibits intestinal absorption).

Q: Which of the following gives a positive reaction with Ferric chloride?

None of the above. The **Ferric Chloride ($FeCl_3$) test** is a classic biochemical screening tool used to detect specific metabolites in urine, typically those containing phenolic or keto-acid groups. ### Why "None of the above" is the correct answer: While options A, B, and C are all associated with positive Ferric chloride tests in traditional textbooks, the question likely hinges on **specificity and modern diagnostic standards**. In many competitive exams, if the question implies a *pathognomonic* or *specific* diagnostic test, Ferric chloride is often considered outdated or non-specific. However, from a purely biochemical standpoint, all three conditions *can* produce a color change. If "None of the above" is the designated key, it emphasizes that this test is a non-specific screening tool and not a definitive diagnostic marker for these complex metabolic disorders. ### Analysis of Options: * **Phenylketonuria (PKU):** Classically gives a **Blue-Green** reaction due to the presence of Phenylpyruvic acid. * **Alkaptonuria:** Classically gives a **Transient Blue-Green** reaction (which quickly fades) due to Homogentisic acid. * **Maple Syrup Urine Disease (MSUD):** Classically gives a **Navy Blue** reaction due to Alpha-keto acids (specifically alpha-ketoisovalerate, etc.). ### High-Yield Clinical Pearls for NEET-PG: * **PKU:** Deficiency of Phenylalanine Hydroxylase; presents with "mousy/musty" odor and intellectual disability. * **Alkaptonuria:** Deficiency of Homogentisic Acid Oxidase; characterized by urine that turns black on standing and ochronosis (pigmentation of cartilage). * **MSUD:** Deficiency of Branched-chain alpha-keto acid dehydrogenase; urine smells like burnt sugar/maple syrup. * **Tyrosinemia:** Gives a **Transient Green** reaction with Ferric chloride. * **Salicylate Poisoning:** Gives a **Stable Purple** reaction (important differential for PKU).

Q: In alkaptonuria, there is increased pigmentation in all of the following locations, EXCEPT:

Nose. **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase**. This leads to the accumulation of **homogentisic acid (HGA)**, which undergoes oxidation and polymerization to form a melanin-like black pigment. This process is known as **ochronosis**. ### Why "Nose" is the Correct Answer: While ochronosis affects connective tissues throughout the body, the pigmentation is most prominent in areas where cartilage is superficial or where there is high exposure to sweat and light. In alkaptonuria, the **ear (auricular cartilage)** and the **eyes (sclera)** are classic sites of visible pigment deposition. The nose, despite having a cartilaginous framework, does not typically manifest the characteristic dark blue-black pigmentation seen in the ears or eyes, making it the "except" option in this clinical context. ### Analysis of Incorrect Options: * **Eyes (A):** Pigmentation (Osler’s sign) typically appears as slate-gray or black spots on the sclera, usually midway between the cornea and the inner/outer canthus. * **Ear (C) & Auricular Cartilage (D):** The cartilage of the pinna is one of the earliest and most common sites for ochronotic changes. The ears appear thickened and develop a characteristic blue-black or "lead-gray" discoloration. ### NEET-PG High-Yield Pearls: * **Triad of Alkaptonuria:** 1. Homogentisic aciduria (urine turns black on standing/alkalinization), 2. Ochronosis (tissue pigmentation), and 3. Arthritis (large joint involvement). * **Biochemical Test:** Benedict’s test is positive (HGA is a reducing agent), and the Ferric Chloride test gives a transient deep blue color. * **Dietary Management:** Restriction of **Phenylalanine and Tyrosine** intake. **Nitisinone** is the drug of choice as it inhibits 4-hydroxyphenylpyruvate dioxygenase, reducing HGA production.

Q: Which of the following inborn errors of metabolism presents with porphyria-like clinical features?

Tyrosinemia type I. **Explanation:** **Tyrosinemia Type I** (Hepatorenal Tyrosinemia) is caused by a deficiency of the enzyme **Fumarylacetoacetate Hydrolase (FAH)**. This deficiency leads to the accumulation of fumarylacetoacetate, which is converted into **Succinylacetone**. Succinylacetone is a potent inhibitor of **ALA Dehydratase** (Porphobilinogen Synthase), an enzyme in the heme synthesis pathway. This inhibition mimics **ADP (ALA Dehydratase Deficiency) Porphyria**, leading to the accumulation of delta-aminolevulinic acid (ALA). Consequently, patients present with "porphyria-like" neurological crises, including abdominal pain and peripheral neuropathy. **Analysis of Incorrect Options:** * **Phenylketonuria (PKU):** Caused by Phenylalanine Hydroxylase deficiency. It presents with intellectual disability, seizures, and a "mousy" odor, but does not interfere with heme synthesis. * **Alkaptonuria:** Caused by Homogentisate Oxidase deficiency. It presents with ochronosis (darkening of tissues), dark urine on standing, and arthritis, but no porphyria-like symptoms. * **Metachromatic Leukodystrophy:** A lysosomal storage disorder (Arylsulfatase A deficiency). While it involves neurological regression, the pathology is due to sulfatide accumulation in myelin, not heme pathway inhibition. **NEET-PG High-Yield Pearls:** * **Pathognomonic Marker:** Succinylacetone in urine is diagnostic for Tyrosinemia Type I. * **Clinical Triad:** Liver failure (cirrhosis/HCC), renal tubular dysfunction (Fanconi syndrome), and porphyric crises. * **Treatment:** **Nitisinone (NTBC)**, which inhibits 4-hydroxyphenylpyruvate dioxygenase to prevent the formation of toxic succinylacetone.

Q: Which statement is false regarding Hurler syndrome?

X-linked inheritance. **Explanation:** **Hurler Syndrome (MPS IH)** is the most severe form of Mucopolysaccharidosis Type I. The correct answer is **A** because Hurler syndrome follows an **Autosomal Recessive** inheritance pattern, not X-linked. 1. **Why Option A is the correct (false) statement:** Hurler syndrome is caused by a deficiency of the enzyme **$\alpha$-L-iduronidase**, encoded by a gene on chromosome 4. Among the Mucopolysaccharidoses, only **Hunter syndrome (MPS II)** is X-linked recessive (mnemonic: *"The Hunter aims for the X"*). All other MPS types, including Hurler, are autosomal recessive. 2. **Why other options are incorrect (true for Hurler):** * **Mental Retardation (B):** Unlike milder forms (Scheie syndrome), Hurler syndrome involves significant progressive neurodegeneration leading to developmental delay and cognitive impairment. * **Joint Stiffness (C):** Accumulation of dermatan sulfate and heparan sulfate in connective tissues leads to restricted joint mobility and contractures. * **Coarse Facial Features (D):** This is a hallmark sign (Gargoylism), characterized by a flat nasal bridge, thick lips, and an enlarged tongue due to GAG deposition. **High-Yield Clinical Pearls for NEET-PG:** * **Corneal Clouding:** Present in Hurler syndrome but **absent** in Hunter syndrome (a common "distractor" in exams). * **Biochemical Marker:** Increased urinary excretion of **Dermatan sulfate** and **Heparan sulfate**. * **Diagnosis:** Confirmed by enzyme assay in leukocytes or fibroblasts. * **Treatment:** Enzyme Replacement Therapy (Laronidase) and Hematopoietic Stem Cell Transplant (HSCT).

Q: Which of the following statements is true regarding the management of Phenylketonuria?

Limiting the substrate for the deficient enzyme. **Explanation:** **Phenylketonuria (PKU)** is an autosomal recessive disorder caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**. This enzyme normally converts the amino acid Phenylalanine into Tyrosine. **1. Why Option A is Correct:** In PKU, the metabolic block leads to the toxic accumulation of **Phenylalanine (the substrate)** in the blood and brain. High levels of phenylalanine are neurotoxic and lead to intellectual disability. Therefore, the primary management strategy is **limiting the substrate** by providing a diet low in Phenylalanine. This prevents the buildup of toxic metabolites like phenylpyruvate and phenyllactate. **2. Analysis of Incorrect Options:** * **Option B:** While Tyrosine becomes an "essential" amino acid in PKU and is supplemented, the statement "supplementing missing amino acids" is too vague. The hallmark of management is restriction, not just supplementation. * **Option C:** Although research is ongoing, **gene therapy** is not yet a standard or "successfully usable" clinical treatment for PKU in routine practice. * **Option D:** While regular cow's milk is high in phenylalanine and must be restricted, the child **cannot completely avoid milk**. Infants require specialized Phenylalanine-free medical formulas to ensure they receive adequate protein for growth. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mousy/Musty Odor:** Due to phenylacetic acid in sweat and urine. * **Hypopigmentation:** Phenylalanine inhibits Tyrosinase, leading to fair skin and blue eyes. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high phenylalanine levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus. * **Sapropterin:** A synthetic form of BH4 (cofactor) used in BH4-responsive variants of PKU.

Q: Which one of the following is not a mitochondrial disorder?

Agammaglobulinemia. ### Explanation **1. Why Agammaglobulinemia is the correct answer:** Agammaglobulinemia (specifically Bruton’s Agammaglobulinemia) is an **X-linked recessive** primary immunodeficiency disorder. It is caused by a mutation in the **BTK gene** (Bruton Tyrosine Kinase), which is essential for B-cell maturation. It is a nuclear genetic defect affecting the immune system, not a disorder of the mitochondrial DNA (mtDNA) or the respiratory chain. **2. Why the other options are incorrect (Mitochondrial Disorders):** Mitochondrial disorders typically affect organs with high energy demands (brain, muscle, heart) and often exhibit **maternal inheritance**. * **Leigh Syndrome (Subacute Necrotizing Encephalomyelopathy):** A severe neurological disorder characterized by psychomotor regression. It can be caused by mutations in both mtDNA and nuclear DNA affecting the mitochondrial respiratory chain. * **Pearson Syndrome:** A rare multisystem condition involving sideroblastic anemia and exocrine pancreatic dysfunction. It is caused by a **large-scale deletion of mtDNA**. * **NARP (Neurogenic muscle weakness, Ataxia, and Retinitis Pigmentosa):** A classic mitochondrial syndrome caused by a mutation in the **ATPase 6 gene** of mtDNA. **3. High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother. All children of an affected mother are at risk, but an affected father cannot pass it to his offspring. * **Heteroplasmy:** The presence of a mixture of normal and mutated mtDNA within a single cell, which explains the clinical variability in mitochondrial diseases. * **Ragged Red Fibers:** A hallmark histological finding on Gömöri trichome stain in many mitochondrial myopathies (e.g., MERRF). * **Common Triad:** Think of mitochondrial disease when a patient presents with a combination of **Encephalopathy, Lactic Acidosis, and Myopathy.**

Q: All are true about Fabry disease, EXCEPT:

Retinitis pigmentosa is well described in Fabry disease. **Explanation:** **Fabry disease** is a lysosomal storage disorder caused by the deficiency of the enzyme **$\alpha$-galactosidase A**, leading to the systemic accumulation of **globotriaosylceramide (ceramide trihexoside)**. 1. **Why Option C is the correct answer (The "EXCEPT"):** Retinitis pigmentosa is **not** a feature of Fabry disease. The characteristic ocular finding in Fabry disease is **Cornea Verticillata** (vortex keratopathy/whorl-like corneal opacities), which does not typically affect vision. Other ocular signs include posterior subcapsular cataracts and tortuous conjunctival vessels. Retinitis pigmentosa is more commonly associated with disorders like Refsum disease or Usher syndrome. 2. **Analysis of other options:** * **Option A:** Fabry disease is unique among sphingolipidoses (most are autosomal recessive) because it is an **X-linked recessive** disorder. * **Option B:** The accumulation of glycosphingolipids in the vascular endothelium leads to narrowing of the lumen, causing **premature atherosclerosis**, myocardial infarction, and stroke at a young age. * **Option D:** The primary substrate that accumulates in the lysosomes of nerves, kidneys, and blood vessels is **ceramide trihexoside** (also known as $Gb_3$). **High-Yield Clinical Pearls for NEET-PG:** * **Early Signs:** Episodic peripheral neuropathy (**acroparesthesia**—burning pain in hands/feet), **angiokeratomas** (dark red skin spots in bathing trunk distribution), and **hypohidrosis** (decreased sweating). * **Late Complications:** Progressive renal failure (leading cause of death) and hypertrophic cardiomyopathy. * **Diagnosis:** Low $\alpha$-galactosidase A activity in leukocytes; confirmed by genetic testing. * **Treatment:** Enzyme Replacement Therapy (ERT) with Agalsidase beta.

Question 1

Metachromatic leukodystrophy is due to deficiency of which enzyme?

Accepted Answer

Arylsulfatase

Answer

Hexosaminidase A

Answer

Hexosaminidase B

Answer

Ceramidase

Question 2

Wilson's disease is caused by a defect in which gene?

Accepted Answer

ATP7B mutation

Answer

ATP7A mutation

Answer

Ceruloplasmin

Answer

ATP7C mutation

Question 3

Which of the following gives a positive reaction with Ferric chloride?

Accepted Answer

None of the above

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Maple syrup urine disease

Question 4

In alkaptonuria, there is increased pigmentation in all of the following locations, EXCEPT:

Accepted Answer

Nose

Answer

Eyes

Answer

Ear

Answer

Auricular cartilage

Question 5

Which of the following inborn errors of metabolism presents with porphyria-like clinical features?

Accepted Answer

Tyrosinemia type I

Answer

Phenylketonuria

Answer

Alkaptonuria

Answer

Metachromatic leukodystrophy

Question 6

Which statement is false regarding Hurler syndrome?

Accepted Answer

X-linked inheritance

Answer

Mental retardation

Answer

Joint stiffness

Answer

Coarse facial features

Question 7

A 10-year-old child presents with symptoms suggestive of pellagra, including chronic diarrhea, a red scaly rash, and mild cerebellar ataxia. The child's diet is adequate in protein and niacin. A sister has a similar presentation. Chemical analysis of the patient's urine shows large amounts of free amino acids. What is the most likely diagnosis?

Accepted Answer

Hartnup disease

Answer

Alkaptonuria

Answer

Carcinoid syndrome

Answer

Ehlers-Danlos syndrome

Question 8

Which of the following statements is true regarding the management of Phenylketonuria?

Accepted Answer

Limiting the substrate for the deficient enzyme

Answer

Supplementing the missing amino acids through diet

Answer

Gene therapy is a successfully usable treatment

Answer

The child should avoid milk consumption

Question 9

Which one of the following is not a mitochondrial disorder?

Accepted Answer

Agammaglobulinemia

Answer

Leigh syndrome

Answer

Pearson syndrome

Answer

NARP

Question 10

All are true about Fabry disease, EXCEPT:

Accepted Answer

Retinitis pigmentosa is well described in Fabry disease

Answer

X-linked recessive disorder

Answer

Premature atherosclerosis is seen

Answer

Accumulation of ceramide trihexoside in nerves and blood vessels

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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