Genetic Disorders and Biochemical Pathology Practice Questions

Q: MRP2 associated with which of the following?

Dubin-Johnson syndrome. No changes were made to the original explanation because the available references provided insufficient evidence to support the specific claims about the MRP2 gene and the characteristic 'dark liver' appearance of Dubin-Johnson syndrome. While the references discuss hyperbilirubinemia and mentions Gilbert and Crigler-Najjar syndromes [1], they do not explicitly detail the MRP2 mutation or the pathology of Dubin-Johnson and Rotor syndromes necessary for high-accuracy medical citation [2].

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: 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: 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: 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: 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: 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: 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.

Q: Which of the following is caused by congenital 17 hydroxylase deficiency:

Hypertension. ***Hypertension*** - **17-hydroxylase deficiency** causes a unique combination among congenital adrenal hyperplasia (CAH) variants: **hypertension with sexual infantilism**. - The enzyme block shunts steroid synthesis toward the mineralocorticoid pathway, leading to excessive **deoxycorticosterone (DOC)** and **corticosterone** production. - Elevated DOC causes **sodium retention**, **volume expansion**, and **hypertension** — this distinguishes it from other CAH forms (like 21-hydroxylase deficiency, which causes hypotension). - **Hypertension is the key diagnostic feature** that differentiates this from other causes of sexual infantilism. *Hypokalemia* - Also a **characteristic feature** of 17-hydroxylase deficiency, caused by the same mineralocorticoid excess (DOC). - The elevated DOC promotes **potassium wasting** in the renal tubules. - However, **hypertension** is typically considered the primary distinguishing feature, with hypokalemia as an associated finding. *Delayed sexual development* - This is a **major manifestation** of 17-hydroxylase deficiency due to impaired synthesis of **both androgens and estrogens**. - Presents as **primary amenorrhea** and absent secondary sexual characteristics in 46,XX females. - Presents as **sexual infantilism** in 46,XY individuals with female external genitalia. - While this is indeed caused by the deficiency, the question focuses on the **distinguishing biochemical feature** (hypertension with hypokalemia). *Ambiguous genitalia in males* - **Genetically male (46,XY) individuals** with 17-hydroxylase deficiency typically have **female or ambiguous external genitalia** due to lack of testosterone synthesis. - This represents **undervirilization** rather than virilization. - Like delayed sexual development, this is a cardinal feature, but **hypertension** is the biochemical hallmark that distinguishes this CAH variant from others.

Question 1

MRP2 associated with which of the following?

Accepted Answer

Dubin-Johnson syndrome

Answer

Rotor syndrome

Answer

Crigler-Najjar syndrome

Answer

Gilbert syndrome

Question 2

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 3

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 4

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 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

Which enzyme deficiency causes Lesch-Nyhan syndrome?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Answer

Xanthine oxidase

Answer

Adenine phosphoribosyltransferase (APRT)

Answer

AMP deaminase

Question 7

Zellweger syndrome is due to the absence of

Accepted Answer

Peroxisomes

Answer

Lysosomes

Answer

Mitochondria

Answer

Nucleus

Question 8

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 9

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

Question 10

Which of the following is caused by congenital 17 hydroxylase deficiency:

Accepted Answer

Hypertension

Answer

Delayed sexual development

Answer

Ambiguous genitalia in males

Answer

Hypokalemia

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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