Genetic Disorders and Biochemical Pathology — MCQs

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?

Q428Easy

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

Q429Easy

Which one of the following is not a mitochondrial disorder?

Q430Medium

All are true about Fabry disease, EXCEPT:

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

Question 421: Metachromatic leukodystrophy is due to deficiency of which enzyme?

A. Hexosaminidase A
B. Hexosaminidase B
C. Ceramidase
D. Arylsulfatase (Correct Answer)

Explanation: **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.

Question 422: Wilson's disease is caused by a defect in which gene?

A. ATP7A mutation
B. ATP7B mutation (Correct Answer)
C. Ceruloplasmin
D. ATP7C mutation

Explanation: **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).

Question 423: Which of the following gives a positive reaction with Ferric chloride?

A. Phenylketonuria
B. Alkaptonuria
C. Maple syrup urine disease
D. None of the above (Correct Answer)

Explanation: 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).

Question 424: In alkaptonuria, there is increased pigmentation in all of the following locations, EXCEPT:

A. Eyes
B. Nose (Correct Answer)
C. Ear
D. Auricular cartilage

Explanation: **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.

Question 425: Which of the following inborn errors of metabolism presents with porphyria-like clinical features?

A. Phenylketonuria
B. Tyrosinemia type I (Correct Answer)
C. Alkaptonuria
D. Metachromatic leukodystrophy

Explanation: **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.

Question 426: Which statement is false regarding Hurler syndrome?

A. X-linked inheritance (Correct Answer)
B. Mental retardation
C. Joint stiffness
D. Coarse facial features

Explanation: **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).

Question 427: 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?

A. Alkaptonuria
B. Carcinoid syndrome
C. Ehlers-Danlos syndrome
D. Hartnup disease (Correct Answer)

Explanation: **Explanation:** The clinical presentation of **pellagra-like symptoms** (dermatitis, diarrhea, and ataxia) in a child with an adequate diet and a positive family history strongly suggests **Hartnup disease**. **1. Why Hartnup Disease is Correct:** Hartnup disease is an autosomal recessive disorder caused by a mutation in the **SLC6A19 gene**, which encodes a neutral amino acid transporter in the proximal renal tubules and intestinal mucosa. This leads to the malabsorption and excessive urinary loss (aminoaciduria) of **neutral amino acids**, most notably **Tryptophan**. Since Tryptophan is a precursor for **Niacin (Vitamin B3)** synthesis, its deficiency results in "secondary pellagra." The cerebellar ataxia is a classic neurological manifestation of this specific metabolic derangement. **2. Why Other Options are Incorrect:** * **Alkaptonuria:** Caused by a deficiency of homogentisate oxidase. It presents with dark urine (on standing), ochronosis (pigmentation), and arthritis, not pellagra-like symptoms. * **Carcinoid Syndrome:** While it can cause pellagra (due to Tryptophan being diverted to Serotonin synthesis), it typically presents in adults with flushing, wheezing, and right-sided heart valves lesions. * **Ehlers-Danlos Syndrome:** A connective tissue disorder characterized by joint hypermobility and skin hyperextensibility; it has no association with aminoaciduria or niacin deficiency. **3. NEET-PG High-Yield Pearls:** * **Biochemical Hallmark:** Neutral aminoaciduria (Tryptophan, Alanine, Valine, Leucine, Isoleucine, Phenylalanine, Tyrosine). * **Diagnosis:** High levels of neutral amino acids in urine (chromatography) but **normal** levels of proline, hydroxyproline, and arginine (distinguishes it from other aminoacidurias). * **Treatment:** High-protein diet and **Nicotinamide** supplementation. * **The "3 Ds" of Pellagra:** Dermatitis, Diarrhea, Dementia (or Death). In Hartnup, cerebellar ataxia is the common neurological finding instead of frank dementia.

Question 428: Which of the following statements is true regarding the management of Phenylketonuria?

A. Limiting the substrate for the deficient enzyme (Correct Answer)
B. Supplementing the missing amino acids through diet
C. Gene therapy is a successfully usable treatment
D. The child should avoid milk consumption

Explanation: **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.

Question 429: Which one of the following is not a mitochondrial disorder?

A. Leigh syndrome
B. Pearson syndrome
C. NARP
D. Agammaglobulinemia (Correct Answer)

Explanation: ### 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.**

Question 430: All are true about Fabry disease, EXCEPT:

A. X-linked recessive disorder
B. Premature atherosclerosis is seen
C. Retinitis pigmentosa is well described in Fabry disease (Correct Answer)
D. Accumulation of ceramide trihexoside in nerves and blood vessels

Explanation: **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.

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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