Genetic Disorders and Biochemical Pathology Practice Questions

Q: A 4-year-old boy from a first-degree consanguineous couple presents with darkening of the urine to an almost black color when left standing. He has a normal sibling and no other medical problems. His growth and development are normal. Which of the following metabolites is most likely to be elevated in this patient?

Homogentisate. ***Correct: Homogentisate*** - The clinical presentation of urine darkening to an almost black color upon standing, especially in a child from a consanguineous union, is classic for **alkaptonuria**. - **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of homogentisate 1,2-dioxygenase, leading to the accumulation of **homogentisate** in the body. - The homogentisic acid polymerizes upon exposure to air, causing the characteristic dark discoloration of urine. - Patients are typically asymptomatic in childhood; **ochronosis** (pigment deposition in connective tissues) and arthritis develop in adulthood. *Incorrect: Methylmalonate* - Elevated **methylmalonate** is characteristic of **methylmalonic aciduria**, a disorder of branched-chain amino acid metabolism or vitamin B12 deficiency. - This condition typically presents with metabolic acidosis, developmental delays, and failure to thrive, which are not described in this patient. *Incorrect: Phenylpyruvate* - Elevated **phenylpyruvate** is a hallmark of **phenylketonuria (PKU)**, an inborn error of metabolism affecting phenylalanine breakdown. - Untreated PKU leads to severe neurological impairment and intellectual disability, which are absent in this child. *Incorrect: α-Ketoisovalerate* - Elevated **α-Ketoisovalerate** is one of the branched-chain keto acids found in **maple syrup urine disease (MSUD)**. - MSUD presents in infancy with poor feeding, lethargy, and a characteristic maple syrup odor in the urine, not darkening urine.

Q: Which of the following is an example of an X-linked disorder?

Color blindness. ***Color blindness*** - **Color blindness**, particularly red-green color blindness, is a classic example of an **X-linked recessive disorder**. - It results from mutations in genes encoding **photopigments** located on the X chromosome, affecting more males than females. - Represents one of the most commonly cited examples of X-linked inheritance in medical education. *Thalassemia* - **Thalassemia** is an **autosomal recessive disorder** affecting the synthesis of hemoglobin chains (α or β chains). - Not an X-linked condition; mutations are in genes on chromosome 16 (α-thalassemia) or chromosome 11 (β-thalassemia). - Commonly seen in populations from the Mediterranean, Middle East, and Asia. *Azoospermia* - **Azoospermia** (absence of sperm in ejaculate) is a clinical finding, not a specific genetic disorder. - Can result from various genetic causes including **Y-chromosome microdeletions** and **autosomal mutations** (e.g., CFTR gene in congenital bilateral absence of vas deferens). - Not classified as an X-linked disorder. *Sickle cell anemia* - **Sickle cell anemia** is an **autosomal recessive disorder** caused by a mutation in the β-globin gene on chromosome 11. - Results from substitution of valine for glutamic acid at position 6 of the β-globin chain (HbS). - Not an X-linked condition; both males and females are equally affected when inheriting two copies of the mutant allele.

Q: Which enzyme deficiency causes Lesch-Nyhan syndrome?

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT). ***Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)*** - **Lesch-Nyhan syndrome** is an X-linked recessive disorder caused by a severe deficiency of the enzyme **HGPRT**. - This deficiency leads to a buildup of **uric acid** due to impaired purine salvage, as well as neurologic dysfunction and self-mutilation. *Xanthine oxidase deficiency* - This deficiency leads to **xanthinuria**, characterized by high levels of xanthine in the urine, which can cause **kidney stones**. - It does not cause the severe neurological and behavioral symptoms seen in Lesch-Nyhan syndrome. *Adenine phosphoribosyltransferase (APRT) deficiency* - Deficiency of **APRT** causes a rare disorder resulting in the overproduction of **2,8-dihydroxyadenine**, which can form renal stones. - While it is involved in purine metabolism, it does not lead to the specific clinical presentation of Lesch-Nyhan syndrome. *AMP deaminase deficiency* - **AMP deaminase deficiency** is a relatively common enzyme defect that can cause exercise-induced myalgia or fatigue. - It affects muscle function and energy metabolism but is not associated with the severe hyperuricemia and neurobehavioral symptoms of Lesch-Nyhan syndrome.

Q: What is the enzyme that is deficient in Phenylketonuria?

Phenylalanine hydroxylase. ***Phenylalanine hydroxylase*** - **Phenylalanine hydroxylase** is the enzyme responsible for converting the amino acid **phenylalanine** into **tyrosine**. - A deficiency in this enzyme leads to the accumulation of **phenylalanine** in the body, causing **phenylketonuria (PKU)**. *Phenylalanine (amino acid)* - **Phenylalanine** is an essential **amino acid** that accumulates in PKU due to the enzyme deficiency, but it is not the deficient enzyme itself. - High levels of **phenylalanine** are toxic to the brain, leading to the clinical manifestations of PKU. *Phenylene* - **Phenylene** is a **bivalent organic radical** derived from benzene, and is not an enzyme or directly related to the metabolic pathway of phenylalanine. - This term is chemically distinct and irrelevant to the enzymatic defect in PKU. *None of the options* - This option is incorrect because **phenylalanine hydroxylase** is indeed the deficient enzyme in PKU, making one of the provided choices correct.

Q: Menkes disease is caused by a deficiency of which protein?

ATP7A (copper-transporting ATPase). ***ATP7A (copper-transporting ATPase)*** - **Menkes disease** is an X-linked recessive disorder caused by a mutation in the **ATP7A gene**, which encodes a copper-transporting ATPase. - This protein is essential for **copper absorption** from the intestines and its transport across cell membranes. *ATP7B (Wilson disease protein)* - Mutations in the **ATP7B gene** cause **Wilson disease**, characterized by **copper accumulation** in the liver, brain, and other organs due to impaired copper excretion. - Unlike Menkes disease, Wilson disease involves *too much* copper in tissues, not a deficiency due to poor absorption. *Ceruloplasmin* - **Ceruloplasmin** is a copper-carrying protein that transports copper in the blood and also acts as an oxidase. - While deficiencies in ceruloplasmin can lead to **aceruloplasminemia**, a disorder of iron metabolism, it is not the primary defect in Menkes disease. *Copper-zinc superoxide dismutase* - **Copper-zinc superoxide dismutase (SOD1)** is an enzyme that plays a crucial role in eliminating harmful **reactive oxygen species**. - Mutations in SOD1 are associated with some forms of **amyotrophic lateral sclerosis (ALS)**, not Menkes disease.

Q: Zellweger syndrome is due to the absence of

Peroxisomes. ***Peroxisomes*** - **Zellweger syndrome** is an **autosomal recessive disorder** characterized by the absence or severe reduction of functional **peroxisomes**. - This leads to the accumulation of very long-chain fatty acids (VLCFAs) and branched-chain fatty acids, primarily affecting the **brain**, **liver**, and **kidneys**. *Lysosomes* - Lysosomes are responsible for the degradation of waste materials and cellular debris. - Absence or dysfunction of **lysosomal enzymes** causes storage diseases like **Tay-Sachs disease** or **Gaucher disease**, not Zellweger syndrome. *Mitochondria* - Mitochondria are the primary sites of **cellular respiration** and ATP production. - Disorders of mitochondria, such as **Leigh syndrome** or **MELAS syndrome**, affect energy metabolism and have different clinical presentations. *Nucleus* - The nucleus contains the cell's genetic material and controls cell growth and reproduction. - While genetic mutations underlie Zellweger syndrome, the **nucleus itself is present** and functional in affected individuals.

Q: In Wilson's disease, there is increased urinary excretion of which of the following substances?

Copper. ***Copper*** - Wilson's disease is characterized by a defect in **copper metabolism** due to mutations in the **ATP7B gene**, leading to impaired biliary copper excretion and accumulation in various tissues, including the **liver, brain, eyes (Kayser-Fleischer rings), and kidneys**. - There is **increased urinary excretion of copper** as the body attempts to eliminate the excess copper; **24-hour urinary copper >100 μg/24h** (often >200 μg/24h) is a key diagnostic finding in Wilson's disease. - This increased urinary copper excretion is one of the most important laboratory tests for diagnosing this condition. *Ceruloplasmin* - **Ceruloplasmin** is a copper-carrying protein in the blood, and its **serum levels are typically decreased (<20 mg/dL)** in Wilson's disease, not increased in urine. - Low serum ceruloplasmin is a diagnostic marker for Wilson's disease, reflecting the impaired incorporation of copper into this protein due to the ATP7B defect. - Ceruloplasmin itself is not significantly excreted in urine. *Serine* - **Serine** is an amino acid and its urinary excretion is not specifically affected in **Wilson's disease**. - Abnormalities in amino acid excretion are more characteristic of other metabolic disorders, such as Fanconi syndrome or aminoacidurias. *Phosphotyrosine* - **Phosphotyrosine** is a modified amino acid residue involved in cell signaling pathways and is completely unrelated to **copper metabolism** or Wilson's disease. - Its presence or excretion in urine is not a diagnostic marker for this condition.

Q: Fish odor syndrome is caused by deficiency of which enzyme?

Flavin-containing monooxygenase 3 (FMO3). ***Flavin-containing monooxygenase 3 (FMO3)*** - **Fish odor syndrome**, also known as **trimethylaminuria**, results from a genetic deficiency or reduced activity of the **FMO3 enzyme**. - **FMO3** is responsible for metabolizing **trimethylamine (TMA)**, a pungent compound produced by gut bacteria, into its odorless N-oxide form (TMAO). When **FMO3** is dysfunctional, **TMA** accumulates and is excreted in bodily secretions, causing a distinctive fishy odor. *Trimethylamine dehydrogenase* - This is a **distractor enzyme** that does not play a significant role in trimethylamine metabolism in humans. - **Trimethylamine (TMA)** is primarily produced by **gut bacterial metabolism** of dietary choline, carnitine, and betaine, not by human enzymatic dehydrogenation. - This enzyme is not related to the pathogenesis of fish odor syndrome. *Cytochrome P450 oxidase* - **Cytochrome P450 enzymes** are a large superfamily of enzymes primarily involved in the metabolism of various xenobiotics and endogenous compounds, including many drugs. - While important for detoxification, they are not directly involved in the metabolism of **trimethylamine** or the pathogenesis of fish odor syndrome. *Monoamine oxidase* - **Monoamine oxidase** enzymes (MAO-A and MAO-B) are crucial for the metabolism of monoamines, such as neurotransmitters like serotonin, dopamine, and norepinephrine. - They play no direct role in the metabolism of **trimethylamine** or the etiology of fish odor syndrome.

Q: Which of the following is a metabolic disorder inherited in an X-linked manner?

Adrenoleukodystrophy. ***Adrenoleukodystrophy*** - **Adrenoleukodystrophy (ALD)** is an **X-linked recessive disorder** that affects the metabolism of very long-chain fatty acids (VLCFAs). - It leads to the demyelination of nerve cells in the brain and spinal cord, as well as adrenal gland insufficiency. *Phenylketonuria* - **Phenylketonuria (PKU)** is an **autosomal recessive metabolic disorder** caused by a defect in the enzyme phenylalanine hydroxylase. - It results in the accumulation of phenylalanine, leading to intellectual disability if not treated with a specialized diet. *Duchenne muscular dystrophy* - While **Duchenne muscular dystrophy (DMD)** is indeed an **X-linked recessive disorder**, it is primarily a muscle disorder, not a metabolic disorder in the classic sense. - It involves a mutation in the **dystrophin gene**, leading to progressive muscle degeneration and weakness. *Marfan syndrome* - **Marfan syndrome** is an **autosomal dominant disorder** affecting connective tissue. - It is caused by a mutation in the **FBN1 gene**, which codes for fibrillin-1, and primarily affects the skeletal, ocular, and cardiovascular systems.

Q: Low serum copper due to ATP7A gene mutation is seen in?

Menkes disease. ***Menkes disease*** - **Menkes disease** is an X-linked recessive disorder characterized by a mutation in the **ATP7A gene**, leading to impaired copper transport from the intestine. - This results in a **systemic copper deficiency**, which manifests as **low serum copper** and ceruloplasmin levels, neurological degeneration, and connective tissue abnormalities. *Wilson disease* - **Wilson disease** is caused by a mutation in the **ATP7B gene**, not ATP7A, which leads to impaired biliary excretion of copper. - This results in **copper accumulation** in the liver, brain, and other organs, causing liver disease, neurological symptoms, and Kayser-Fleischer rings. *Gilbert's disease* - **Gilbert's disease** is a common, mild liver disorder characterized by **intermittent unconjugated hyperbilirubinemia**, typically due to reduced activity of the enzyme UDP-glucuronosyltransferase. - It is not related to copper metabolism or mutations in the ATP7A gene. *Dubin-Johnson syndrome* - **Dubin-Johnson syndrome** is an autosomal recessive disorder characterized by a defect in the excretion of conjugated bilirubin from hepatocytes into the bile, leading to **conjugated hyperbilirubinemia**. - It is associated with mutations in the **MRP2 gene (ABCC2)** and is unrelated to copper metabolism.

Question 1

A 4-year-old boy from a first-degree consanguineous couple presents with darkening of the urine to an almost black color when left standing. He has a normal sibling and no other medical problems. His growth and development are normal. Which of the following metabolites is most likely to be elevated in this patient?

Accepted Answer

Homogentisate

Answer

Methylmalonate

Answer

Phenylpyruvate

Answer

α-Ketoisovalerate

Question 2

Which of the following is an example of an X-linked disorder?

Accepted Answer

Color blindness

Answer

Thalassemia

Answer

Azoospermia

Answer

Sickle cell anemia

Question 3

Which enzyme deficiency causes Lesch-Nyhan syndrome?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Answer

Xanthine oxidase

Answer

Adenine phosphoribosyltransferase (APRT)

Answer

AMP deaminase

Question 4

What is the enzyme that is deficient in Phenylketonuria?

Accepted Answer

Phenylalanine hydroxylase

Answer

Phenylene

Answer

Phenylalanine (amino acid)

Answer

None of the options

Question 5

Menkes disease is caused by a deficiency of which protein?

Accepted Answer

ATP7A (copper-transporting ATPase)

Answer

ATP7B (Wilson disease protein)

Answer

Ceruloplasmin

Answer

Copper-zinc superoxide dismutase

Question 6

Zellweger syndrome is due to the absence of

Accepted Answer

Peroxisomes

Answer

Lysosomes

Answer

Mitochondria

Answer

Nucleus

Question 7

In Wilson's disease, there is increased urinary excretion of which of the following substances?

Accepted Answer

Copper

Answer

Phosphotyrosine

Answer

Serine

Answer

Ceruloplasmin

Question 8

Fish odor syndrome is caused by deficiency of which enzyme?

Accepted Answer

Flavin-containing monooxygenase 3 (FMO3)

Answer

Trimethylamine dehydrogenase

Answer

Cytochrome P450 oxidase

Answer

Monoamine oxidase

Question 9

Which of the following is a metabolic disorder inherited in an X-linked manner?

Accepted Answer

Adrenoleukodystrophy

Answer

Duchenne muscular dystrophy

Answer

Phenylketonuria

Answer

Marfan syndrome

Question 10

Low serum copper due to ATP7A gene mutation is seen in?

Accepted Answer

Menkes disease

Answer

Gilbert's disease

Answer

Wilson disease

Answer

Dubin-Johnson syndrome

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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