Genetic Disorders and Biochemical Pathology Practice Questions

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.

Q: A normal female, whose father is color blind, marries a normal man. What are the chances of their son being color blind?

50%. ***50%*** - The mother is a **carrier** because her father is colorblind, meaning she has one normal X chromosome and one affected X chromosome. - A son inherits his X chromosome from his mother; there is a **50% chance** that he will inherit the X chromosome carrying the colorblindness gene. *25%* - This percentage is typically associated with **autosomal recessive** inheritance patterns, not X-linked traits like colorblindness. - It would imply a different genetic setup for the parents than described, such as both parents being carriers for an autosomal recessive condition. *75%* - This probability would suggest a more complex genetic scenario or a condition with **incomplete penetrance** or a dominant inheritance pattern, which does not apply to X-linked recessive colorblindness in this context. - It does not align with the mendelian inheritance pattern for X-linked recessive traits when the mother is a carrier and the father is unaffected. *No chance* - This would only be true if the mother was **not a carrier** of the colorblindness gene. - Since her father was colorblind, she must have inherited his affected X chromosome, making her an obligate carrier.

Q: Hereditary orotic aciduria Type-I is due to deficiency of?

UMP synthase. ***UMP synthase*** - Hereditary orotic aciduria Type-I is caused by a deficiency of the **bifunctional enzyme UMP synthase** (also called UMP synthase complex). - UMP synthase catalyzes two sequential reactions in the *de novo* pyrimidine synthesis pathway: 1. **OPRT activity**: Converts orotate → orotidine 5'-monophosphate (OMP) 2. **ODC activity**: Converts OMP → uridine 5'-monophosphate (UMP) - This is the **most precise and complete answer** as it identifies the actual enzyme complex that is deficient. - **Clinical features**: Megaloblastic anemia, growth retardation, immunodeficiency; responds to oral uridine supplementation. *Orotate phosphoribosyl transferase* - This represents only **one of the two catalytic activities** of the UMP synthase enzyme (the first step). - While this activity is indeed deficient in Type-I orotic aciduria, naming only this activity is **incomplete** because the enzyme has two functions. - This would be a **partial answer** rather than the complete enzyme name. *Orotic acid decarboxylase* - This represents only **the second catalytic activity** of the UMP synthase enzyme (converts OMP to UMP). - Like OPRT, this activity is also deficient, but naming only this component is **incomplete**. - **Type II orotic aciduria** (extremely rare) involves isolated ODC deficiency without OPRT deficiency. *All of the options* - While technically both OPRT and ODC activities are affected in Type-I orotic aciduria, the **standard nomenclature** refers to the deficient enzyme as **"UMP synthase"** - the name of the complete bifunctional enzyme. - In medical terminology and examination context, we identify enzyme deficiencies by the **name of the enzyme complex**, not by listing all its individual catalytic activities. - Therefore, **"UMP synthase"** is the single most accurate and complete answer.

Q: Which organelle is primarily affected in Fabry's disease?

Lysosome. ***Lysosome*** - Fabry's disease is a **lysosomal storage disorder** caused by a deficiency of the enzyme **alpha-galactosidase A**. - This enzyme deficiency leads to the accumulation of **globotriaosylceramide (Gb3)** within lysosomes in various cells throughout the body. *Endoplasmic Reticulum* - The **endoplasmic reticulum** is involved in protein synthesis and folding, and lipid metabolism. - While cellular stress from Gb3 accumulation can indirectly affect the ER, it is not the primary organelle involved in the storage of the accumulated substrate in Fabry's disease. *Golgi apparatus* - The **Golgi apparatus** modifies, sorts, and packages proteins and lipids. - It is not the site of primary pathology or substrate accumulation in lysosomal storage diseases. *Cell membrane* - The **cell membrane** regulates passage of substances into and out of the cell. - While lysosomal dysfunction can ultimately impact overall cell function, the cell membrane itself is not the organelle where the undigested substrate accumulates in Fabry's disease.

Q: Ochronosis is primarily associated with which condition?

Alkaptonuria. ***Alkaptonuria*** - **Ochronosis** is a rare genetic disorder characterized by the accumulation of **homogentisic acid** in connective tissues, leading to a blue-black discoloration; this is the defining feature of alkaptonuria. - Patients with alkaptonuria lack the enzyme **homogentisate 1,2-dioxygenase**, preventing the breakdown of homogentisic acid and causing its buildup. *Hydrochloric acid poisoning* - This condition involves the ingestion of **corrosive acid**, leading to severe burns and tissue damage in the gastrointestinal tract. - It does not cause the characteristic **pigmentation** and systemic connective tissue involvement seen in ochronosis. *Carbolic acid (phenol) poisoning* - **Phenol poisoning** is a toxic emergency characterized by widespread protein denaturation and tissue damage. - It results in systemic toxicity, including cardiovascular and renal effects, but not the **blue-black discoloration of connective tissues** associated with ochronosis. *Oxalic acid poisoning* - **Oxalic acid poisoning** can lead to severe metabolic disturbance, including hypocalcemia due to calcium oxalate formation, and acute renal failure. - It is not associated with the **hereditary metabolic defect** or the unique pattern of pigmentation that defines ochronosis.

Q: If the mother is affected and the father is not, in an autosomal dominant disease, what is the chance of their children being affected?

50% chance of being affected. ***50% chance of being affected*** - In an **autosomal dominant (AD)** disease, only one copy of the **affected gene** is needed for the disease to manifest. - If the mother is affected (heterozygous, Aa) and the father is unaffected (homozygous recessive, aa), their offspring have a **50% chance** of inheriting the dominant affected allele (A) from the mother. *25% chance of being affected* - A 25% chance of being affected is typical for **autosomal recessive** inheritance when both parents are **heterozygous carriers**. - This scenario involves an autosomal dominant trait, where the inheritance pattern is different. *100% chance of being affected* - This would only occur if the mother were **homozygous dominant (AA)** and the father were unaffected (aa), where all offspring would inherit one dominant allele. - In typical autosomal dominant pedigrees, affected individuals are **heterozygous (Aa)**, leading to a 50% risk for each child. *75% chance of being affected* - A 75% chance is not characteristic of simple autosomal dominant inheritance for a cross between an affected heterozygous individual and an unaffected individual. - This outcome would occur if both parents were heterozygous (Aa × Aa), which is not the scenario described in this question.

Q: Hunter syndrome is due to deficiency of which enzyme?

Iduronate-2-sulfatase. ***Iduronate-2-sulfatase*** - **Hunter syndrome** (Mucopolysaccharidosis Type II) is caused by a deficiency of the enzyme **iduronate-2-sulfatase**. - This deficiency leads to the accumulation of **dermatan sulfate** and **heparan sulfate** within lysosomes. *Beta galactosidase* - Deficiency of **beta-galactosidase** is associated with **GM1 gangliosidosis** and **Morquio syndrome type B**. - These conditions present with different clinical features, including neurological degeneration and skeletal abnormalities, not consistent with Hunter syndrome. *Sphingomyelinase* - A deficiency in **sphingomyelinase** causes **Niemann-Pick disease types A and B**. - This leads to the accumulation of **sphingomyelin** in various organs, resulting in hepatosplenomegaly, neurodegeneration, and lung disease. *Hyaluronidase* - **Hyaluronidase** is an enzyme that breaks down hyaluronic acid, which is a component of the extracellular matrix. - While it has various roles in the body and is sometimes used clinically, its deficiency is not associated with Hunter syndrome or a specific lysosomal storage disease.

Q: Conjugated hyperbilirubinemia is seen in which of the following conditions?

Dubin-Johnson syndrome. ***Dubin-Johnson syndrome*** - This is an **autosomal recessive disorder** characterized by a defect in the excretion of **conjugated bilirubin** from hepatocytes into the bile canaliculi due to mutation in the **MRP2 (ABCC2) gene**. - It leads to a **direct (conjugated) hyperbilirubinemia** causing recurrent episodes of **jaundice**, and often a characteristic **black liver** due to melanin-like pigment accumulation. - Typically presents in **adolescence or early adulthood** with mild, fluctuating jaundice. *Gilbert syndrome* - This is a common, benign, **hereditary condition** characterized by intermittently elevated levels of **unconjugated (indirect) bilirubin**. - It results from a partial deficiency of the enzyme **uridine diphosphate glucuronosyltransferase (UGT1A1)**, responsible for bilirubin conjugation. - Presents with mild jaundice during fasting, stress, or illness. *Crigler-Najjar syndrome* - This is a rare, severe, **hereditary disorder** involving a complete (Type I) or near-complete (Type II) deficiency of **UGT1A1 enzyme** activity. - It leads to very high levels of **unconjugated hyperbilirubinemia**, with Type I causing severe **kernicterus** and often death in infancy if untreated. *None of the options* - Since **Dubin-Johnson syndrome** is a well-established cause of conjugated hyperbilirubinemia, this option is incorrect.

Question 1

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 2

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

Question 3

A normal female, whose father is color blind, marries a normal man. What are the chances of their son being color blind?

Accepted Answer

50%

Answer

25%

Answer

75%

Answer

No chance

Question 4

Hereditary orotic aciduria Type-I is due to deficiency of?

Accepted Answer

UMP synthase

Answer

Orotate phosphoribosyl transferase

Answer

Orotic acid decarboxylase

Answer

All of the options

Question 5

Which organelle is primarily affected in Fabry's disease?

Accepted Answer

Lysosome

Answer

Endoplasmic Reticulum

Answer

Golgi apparatus

Answer

Cell membrane

Question 6

Ochronosis is primarily associated with which condition?

Accepted Answer

Alkaptonuria

Answer

Carbolic acid (phenol) poisoning

Answer

Hydrochloric acid poisoning

Answer

Oxalic acid poisoning

Question 7

Which of the following statements about the inheritance of an X-linked recessive trait is true?

Accepted Answer

Carrier mothers have a 50% chance of transmitting the disease to their sons

Answer

Fathers can transmit X-linked traits to their sons

Answer

25% of sons of a carrier mother are affected

Answer

100% of daughters of a diseased father are affected

Question 8

If the mother is affected and the father is not, in an autosomal dominant disease, what is the chance of their children being affected?

Accepted Answer

50% chance of being affected

Answer

25% chance of being affected

Answer

100% chance of being affected

Answer

75% chance of being affected

Question 9

Hunter syndrome is due to deficiency of which enzyme?

Accepted Answer

Iduronate-2-sulfatase

Answer

Beta galactosidase

Answer

Sphingomyelinase

Answer

Hyaluronidase

Question 10

Conjugated hyperbilirubinemia is seen in which of the following conditions?

Accepted Answer

Dubin-Johnson syndrome

Answer

Gilbert syndrome

Answer

Crigler-Najjar syndrome

Answer

None of the options

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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