Genetic Disorders and Biochemical Pathology — MCQs

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

Question 681: Which enzyme is defective in the lysosomal storage disorder known as Gaucher's disease?

A. Hexosaminidase A
B. Sphingomyelinase
C. Glucocerebrosidase (Correct Answer)
D. Galactosidase

Explanation: ***Glucocerebrosidase*** - **Gaucher's disease** is caused by a deficiency in the enzyme **glucocerebrosidase**, leading to the accumulation of glucocerebroside in lysosomes. - This accumulation primarily affects **macrophages**, resulting in hepatosplenomegaly, bone pain, and neurological symptoms in certain types. *Hexosaminidase A* - A deficiency in **hexosaminidase A** is characteristic of **Tay-Sachs disease**, another lysosomal storage disorder. - This deficiency leads to the buildup of **GM2 gangliosides**, primarily affecting the central nervous system and causing neurodegeneration. *Sphingomyelinase* - A defect in **sphingomyelinase** is the underlying cause of **Niemann-Pick disease**, specifically types A and B. - This enzyme deficiency results in the accumulation of **sphingomyelin** in various organs, leading to hepatosplenomegaly and neurodegeneration. *Galactosidase* - While several types of galactosidases exist, a deficiency in **alpha-galactosidase A** is responsible for **Fabry disease**. - This deficiency causes the accumulation of **globotriaosylceramide (Gb3)**, primarily affecting the kidneys, heart, and nervous system, and not associated with Gaucher's.

Question 682: A patient with muscle weakness and cardiomyopathy is diagnosed with Pompe disease. What is the biochemical basis for the symptoms observed in Pompe disease?

A. Accumulation of glycogen in lysosomes (Correct Answer)
B. Accumulation of sphingomyelin
C. Defective β-oxidation of fatty acids
D. Impaired gluconeogenesis

Explanation: ***Accumulation of glycogen in lysosomes*** - Pompe disease is an **autosomal recessive lysosomal storage disorder** caused by a deficiency in acid alpha-glucosidase (GAA), an enzyme responsible for breaking down glycogen in the lysosomes. - The **accumulation of undigested glycogen** within lysosomes, particularly in muscle cells, leads to widespread tissue damage, explaining the patient's muscle weakness and cardiomyopathy. *Accumulation of sphingomyelin* - This is characteristic of **Niemann-Pick disease**, another lysosomal storage disorder, caused by a deficiency in the enzyme **sphingomyelinase**. - While Niemann-Pick disease can affect various organs, its clinical presentation and the specific stored substrate are different from Pompe disease. *Defective β-oxidation of fatty acids* - This defect is associated with conditions like **fatty acid oxidation disorders** (e.g., MCAD deficiency), which can cause muscle weakness and cardiomyopathy but involve problems with fatty acid metabolism, not glycogen. - The primary issue in Pompe disease is the inability to degrade **glycogen**, not fatty acids. *Impaired gluconeogenesis* - Impaired gluconeogenesis leads to **hypoglycemia**, especially during fasting, and is seen in various metabolic disorders affecting liver glucose production. - While some glycogen storage diseases can affect glucose homeostasis, the primary defect in Pompe disease is **lysosomal glycogen degradation**, not the synthesis of glucose from non-carbohydrate sources.

Question 683: In the context of lysosomal storage diseases, what is the consequence of glucocerebrosidase deficiency?

A. Altered lipid metabolism in other pathways
B. Storage of lipid substrates in macrophages (Gaucher cells) (Correct Answer)
C. Impaired glycogen metabolism (seen in Pompe disease)
D. Storage of sphingomyelin in lysosomes (seen in Niemann-Pick disease)

Explanation: ***Storage of lipid substrates in macrophages (Gaucher cells)*** - **Glucocerebrosidase** is an enzyme responsible for breaking down **glucocerebroside**. Its deficiency leads to the accumulation of this lipid. - This accumulation primarily occurs in **macrophages**, forming characteristic **Gaucher cells** with a "wrinkled tissue paper" appearance, leading to **Gaucher disease**. *Impaired glycogen metabolism (seen in Pompe disease)* - Impaired glycogen metabolism is characteristic of **glycogen storage diseases**, such as **Pompe disease**, which involves a deficiency of **acid alpha-glucosidase**. - This deficiency results in the accumulation of **glycogen** in lysosomes, primarily affecting muscle and liver. *Altered lipid metabolism in other pathways* - While glucocerebrosidase deficiency certainly alters lipid metabolism, this option is too general. The specific consequence involves the **storage of glucocerebroside**. - Many lysosomal storage diseases involve altered lipid metabolism, but each is distinct in the specific lipid and enzyme affected. *Storage of sphingomyelin in lysosomes (seen in Niemann-Pick disease)* - The storage of **sphingomyelin** in lysosomes is characteristic of **Niemann-Pick disease**, caused by a deficiency of the enzyme **sphingomyelinase**. - This leads to the accumulation of sphingomyelin in various organs, including the spleen, liver, lungs, and brain.

Question 684: A newborn presents with hypotonia, poor feeding, and a cherry-red spot on the retina. Which lysosomal enzyme deficiency is most likely?

A. Hexosaminidase A (Correct Answer)
B. Arylsulfatase A
C. β-Glucocerebrosidase
D. Sphingomyelinase

Explanation: ***Hexosaminidase A*** - Deficiency of **Hexosaminidase A** leads to **Tay-Sachs disease**, a lysosomal storage disorder characterized by the accumulation of **GM2 ganglioside**. - Classic symptoms include **hypotonia**, **poor feeding**, developmental regression, and a pathognomonic **cherry-red spot on the retina** (present in nearly all cases). - This is the most common cause of cherry-red spot in infancy. *Arylsulfatase A* - Deficiency of **arylsulfatase A** causes **metachromatic leukodystrophy**, which manifests with progressive **demyelination** and neurological decline. - It typically does not present with a cherry-red spot on the retina or the specific constellation of symptoms described. *β-Glucocerebrosidase* - Deficiency of **β-glucocerebrosidase** leads to **Gaucher disease**, characterized by the accumulation of **glucocerebroside** in macrophages. - Symptoms include **hepatosplenomegaly**, bone pain, and cytopenias, but not hypotonia or a cherry-red spot in infancy. *Sphingomyelinase* - Deficiency of **sphingomyelinase** causes **Niemann-Pick disease (Type A and B)**, involving the accumulation of **sphingomyelin**. - Type A can present with hepatosplenomegaly, neurodegeneration, and cherry-red spots (in ~50% of cases), but typically presents with **prominent hepatosplenomegaly** as an early feature, which is not mentioned in this clinical scenario. - The combination of profound hypotonia and poor feeding as presenting features, along with cherry-red spot and **absence of organomegaly**, favors Tay-Sachs over Niemann-Pick.

Question 685: A 30-year-old presents with recurrent kidney stones. Which enzyme deficiency should be investigated for primary hyperoxaluria?

A. Glucose-6-phosphatase
B. Alanine-glyoxylate aminotransferase (Correct Answer)
C. Phenylalanine hydroxylase
D. Lactase

Explanation: ***Alanine-glyoxylate aminotransferase*** - This enzyme (AGT) is deficient in **primary hyperoxaluria type 1 (PH1)**, the most common and severe form of primary hyperoxaluria. - A deficiency in AGT leads to the accumulation of **glyoxylate**, which is then converted into **oxalate**, leading to increased urinary oxalate excretion and recurrent **calcium oxalate kidney stones**. *Glucose-6-phosphatase* - Deficiency of this enzyme is characteristic of **Glycogen Storage Disease Type I (von Gierke disease)**. - It leads to **hypoglycemia**, lactic acidosis, hyperuricemia, and hyperlipidemia, but not primary hyperoxaluria. *Phenylalanine hydroxylase* - A deficiency in this enzyme causes **phenylketonuria (PKU)**, an inborn error of amino acid metabolism. - PKU results in the accumulation of **phenylalanine** and its metabolites, leading to neurological damage if untreated, but is not associated with primary hyperoxaluria. *Lactase* - Lactase deficiency causes **lactose intolerance**, where the body cannot properly digest lactose, a sugar found in milk and dairy products. - Symptoms include bloating, diarrhea, and abdominal cramps after consuming lactose, and it is not related to primary hyperoxaluria or kidney stone formation.

Question 686: A child presents with hepatomegaly, hypoglycemia, and lactic acidosis. Genetic testing confirms a deficiency in glucose-6-phosphatase. What disease does this indicate?

A. Pompe disease
B. Von Gierke disease (Correct Answer)
C. Cori disease
D. Andersen disease

Explanation: ***Von Gierke disease*** - This disease is directly caused by a **deficiency of glucose-6-phosphatase**, leading to the characteristic accumulation of **glycogen in the liver** and kidneys. - The deficiency prevents the release of glucose from glycogen stores, resulting in **hypoglycemia** and a compensatory increase in other metabolic pathways, causing **lactic acidosis**. *Pompe disease* - Caused by a deficiency of **acid α-glucosidase (acid maltase)**, leading to glycogen accumulation primarily in **lysosomes** in muscles, including the heart. - While it involves glycogen storage and can cause **cardiomyopathy** and muscle weakness, it does not typically present with severe hypoglycemia or lactic acidosis in the same way as Von Gierke disease. *Cori disease* - Also known as **Glycogen Storage Disease Type III**, it results from a deficiency in the **glycogen debranching enzyme**. - Patients present with **hepatomegaly**, hypoglycemia, and sometimes muscle weakness, but usually have less severe lactic acidosis than Von Gierke disease and a distinctive short outer chain glycogen structure. *Andersen disease* - Also known as **Glycogen Storage Disease Type IV**, it is caused by a deficiency in the **glycogen branching enzyme**. - This leads to the storage of **abnormal glycogen molecules** (long unbranched chains) primarily in the liver and muscles, often presenting with **cirrhosis** and liver failure, and not typically with severe hypoglycemia or lactic acidosis as the primary metabolic derangement.

Question 687: What is the most common inborn error of metabolism that results in phenylketonuria (PKU)?

A. Phenylalanine hydroxylase deficiency (Correct Answer)
B. Homogentisate oxidase deficiency
C. Branched-chain α-ketoacid dehydrogenase deficiency
D. Ornithine transcarbamylase deficiency

Explanation: ***Phenylalanine hydroxylase deficiency*** - This enzyme is crucial for converting **phenylalanine** to **tyrosine**. Its deficiency leads to the accumulation of phenylalanine, causing phenylketonuria (PKU). - More than 95% of PKU cases are attributed to **mutations** in the gene encoding phenylalanine hydroxylase. *Homogentisate oxidase deficiency* - Deficiency of this enzyme causes **alkaptonuria**, a rare genetic disorder characterized by the accumulation of homogentisic acid. - It is distinct from PKU and does not involve the metabolism of phenylalanine to tyrosine. *Branched-chain α-ketoacid dehydrogenase deficiency* - This enzyme deficiency is associated with **maple syrup urine disease (MSUD)**, a disorder of branched-chain amino acid metabolism. - MSUD affects the metabolism of **leucine, isoleucine, and valine**, not phenylalanine. *Ornithine transcarbamylase deficiency* - This deficiency is the most common **urea cycle disorder**, leading to hyperammonemia. - It primarily affects the body's ability to remove **ammonia** and is unrelated to phenylalanine metabolism.

Question 688: A patient with chronic granulomatous disease has recurrent bacterial infections. Which biochemical pathway is defective?

A. Pentose phosphate pathway (Correct Answer)
B. TCA cycle
C. Beta-oxidation
D. Glycogenolysis

Explanation: ***Pentose phosphate pathway*** - Chronic granulomatous disease (CGD) is characterized by a defect in **NADPH oxidase**, the enzyme responsible for the respiratory burst in phagocytes. - While the defect is in NADPH oxidase itself, the **pentose phosphate pathway** is functionally critical because it is the primary source of **NADPH**, the essential substrate for NADPH oxidase. - Without adequate NADPH production, even functional NADPH oxidase cannot generate superoxide radicals needed to kill bacteria. - This makes the pentose phosphate pathway the most relevant metabolic pathway connected to CGD pathology among the options listed. *TCA cycle* - The **TCA cycle (Krebs cycle)** is central to aerobic respiration and produces ATP through oxidative metabolism. - It is not directly linked to phagocyte respiratory burst or the pathology of CGD. - Defects in the TCA cycle typically lead to severe metabolic dysfunction affecting overall energy production, not specifically immune cell bactericidal function. *Beta-oxidation* - **Beta-oxidation** is the process by which fatty acids are broken down to produce acetyl-CoA and generate energy. - While critical for energy metabolism, it is not implicated in the immune dysfunction or respiratory burst defect seen in CGD. *Glycogenolysis* - **Glycogenolysis** is the breakdown of glycogen into glucose, primarily for maintaining blood glucose levels and providing energy. - Defects in this pathway cause glycogen storage diseases, which present with hepatomegaly, hypoglycemia, or muscle weakness. - These disorders do not involve recurrent bacterial infections due to impaired phagocyte bactericidal function.

Question 689: A patient with severe fasting hypoglycemia and hepatomegaly is diagnosed with von Gierke's disease. Which enzyme deficiency is responsible for this condition?

A. Glycogen synthase
B. Glucose-6-phosphatase (Correct Answer)
C. Debranching enzyme
D. Branching enzyme

Explanation: ***Glucose-6-phosphatase*** - **Von Gierke's disease** (Glycogen Storage Disease Type I) is characterized by a deficiency of **glucose-6-phosphatase**, an enzyme crucial for the final step of both **glycogenolysis** and **gluconeogenesis**. - This deficiency prevents the release of free glucose from the liver into the bloodstream, leading to severe **fasting hypoglycemia** and the accumulation of glycogen in the liver, causing **hepatomegaly**. *Glycogen synthase* - A deficiency in **glycogen synthase** would lead to an inability to synthesize glycogen, resulting in **hypoglycemia** but without the characteristic **hepatomegaly** due to glycogen accumulation. - This condition is known as **Glycogen Storage Disease Type 0**, and it typically presents with fasting hypoglycemia due to insufficient liver glycogen stores, but not excess, unlike Von Gierke's. *Debranching enzyme* - A deficiency in the **debranching enzyme** (α-1,6-glucosidase) is associated with **Cori's disease** (Type III Glycogen Storage Disease). - This leads to the accumulation of **abnormally short glycogen branches**, causing **hepatomegaly** and milder hypoglycemia, but the hypoglycemia is usually not as severe as in Von Gierke's. *Branching enzyme* - A deficiency in the **branching enzyme** (amylo-(1,4 to 1,6)-transglucosidase) is seen in **Andersen's disease** (Type IV Glycogen Storage Disease). - This results in the formation of **unbranched glycogen polymers**, which are less soluble and tend to precipitate, causing **liver cirrhosis** and failure rather than severe fasting hypoglycemia as the primary metabolic issue.

Question 690: A 25-year-old male presents with recurrent kidney stones. Laboratory tests show elevated urine oxalate. Which enzyme deficiency should be considered?

A. Alanine-glyoxylate aminotransferase (Correct Answer)
B. Phenylalanine hydroxylase
C. Uricase
D. Glucocerebrosidase

Explanation: ***Alanine-glyoxylate aminotransferase*** - Deficiency of **alanine-glyoxylate aminotransferase (AGT)** leads to impaired detoxification of **glyoxylate** in the liver, causing its conversion to **oxalate**. - This excess oxalate is then excreted in the urine, leading to **hyperoxaluria** and the formation of **calcium oxalate kidney stones**, consistent with primary hyperoxaluria type 1. *Phenylalanine hydroxylase* - Deficiency of **phenylalanine hydroxylase** causes **phenylketonuria (PKU)**, a metabolic disorder characterized by the accumulation of phenylalanine. - PKU primarily affects neurological development and does not directly lead to recurrent kidney stones from elevated urine oxalate. *Uricase* - **Uricase** converts uric acid to allantoin, a more soluble compound. Humans naturally lack functional uricase. - Elevated uric acid levels can cause **uric acid kidney stones** and gout, but this is distinct from calcium oxalate stones due to hyperoxaluria. *Glucocerebrosidase* - Deficiency of **glucocerebrosidase** causes **Gaucher disease**, a lysosomal storage disorder where glucocerebroside accumulates in macrophages. - Gaucher disease presents with hepatosplenomegaly, bone marrow involvement, and neurological symptoms, not elevated urine oxalate or recurrent kidney stones.

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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