Genetic Disorders and Biochemical Pathology Practice Questions

Q: A 45-year-old man presents with severe abdominal pain and neurological symptoms. His urine turns dark red upon standing. Which enzyme deficiency is most likely responsible for his condition?

Porphobilinogen deaminase. ***Porphobilinogen deaminase*** - Deficiency in **porphobilinogen deaminase (PBG deaminase)**, also known as **hydroxymethylbilane synthase**, is responsible for **acute intermittent porphyria (AIP)**. - This enzyme normally converts **porphobilinogen (PBG)** to **hydroxymethylbilane**; when deficient, there is **upstream accumulation** of **PBG** and **aminolevulinic acid (ALA)**. - These accumulated precursors are **neurotoxic**, causing the characteristic **severe abdominal pain** and **neurological symptoms** (peripheral neuropathy, psychiatric manifestations). - Urinary excretion of **PBG** causes urine to **darken to dark red/port-wine color upon standing** due to oxidation and polymerization. *ALA dehydratase* - Deficiency in **ALA dehydratase** causes **ALA dehydratase deficiency porphyria (ADP)**, also called **Doss porphyria**, which is extremely rare. - It presents similarly to AIP with neurovisceral symptoms, but the accumulation pattern differs (predominantly ALA rather than PBG). *Ferrochelatase* - Deficiency in **ferrochelatase** leads to **erythropoietic protoporphyria (EPP)**, characterized by accumulation of **protoporphyrin IX** in erythrocytes. - The main clinical manifestation is **photosensitivity** with severe pain, burning, and erythema upon sun exposure—not acute abdominal or neurological symptoms. *Coproporphyrinogen oxidase* - Deficiency in **coproporphyrinogen oxidase** causes **hereditary coproporphyria (HCP)**, which can present with neurovisceral attacks similar to AIP. - However, HCP also features **photosensitivity** (unlike AIP), and the classic presentation described with severe abdominal pain, neurological symptoms, and darkening urine most characteristically points to **AIP** caused by PBG deaminase deficiency.

Q: Lesch-Nyhan syndrome is caused by deficiency of which enzyme?

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT). ***Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)*** - **Lesch-Nyhan syndrome** is an **X-linked recessive disorder** caused by a partial or complete deficiency of the enzyme **HGPRT**. - This deficiency leads to an accumulation of **uric acid** and neurological dysfunction, including self-mutilation and dystonia. *Adenine phosphoribosyltransferase (APRT)* - Deficiency of **APRT** primarily leads to the formation of **2,8-dihydroxyadenine stones** in the urinary tract, not Lesch-Nyhan syndrome. - While it is also a salvage pathway enzyme for purines, its deficiency does not result in the severe neurological and behavioral symptoms seen in Lesch-Nyhan. *Xanthine oxidase* - **Xanthine oxidase** is involved in the catabolism of purines, converting **hypoxanthine** to **xanthine** and then to **uric acid**. - Its deficiency (e.g., in xanthinuria) leads to stones composed of **xanthine**, not the neurological and metabolic disturbances of Lesch-Nyhan. *Adenosine deaminase (ADA)* - Deficiency of **ADA** causes **severe combined immunodeficiency (SCID)**, characterized by profound T-cell and B-cell dysfunction. - This enzyme is crucial for the metabolism of adenosine and deoxyadenosine, and its absence leads to lymphocyte toxicity, not the symptoms of Lesch-Nyhan syndrome.

Q: Which disorder is most characteristically linked to the X chromosome?

X-linked color vision deficiency. ***X-linked color vision deficiency*** - This condition is **classically inherited in an X-linked recessive pattern**, meaning the gene responsible for the disorder is located on the **X chromosome**. - Males are more frequently affected because they have only one X chromosome, so a single copy of the mutated gene is sufficient to cause the condition. - This is the **prototypical example** of X-linked recessive inheritance. *Beta-thalassemia* - This is an **autosomal recessive** blood disorder, meaning the genes responsible are located on **non-sex chromosomes** (autosomes). - Both parents must carry the mutated gene for a child to inherit the disorder. *Klinefelter syndrome* - This is a **chromosomal aneuploidy** resulting from the presence of an **extra X chromosome** in males (47,XXY karyotype). - While it involves the X chromosome, it is a **numerical chromosomal abnormality** rather than a genetic disorder linked to a specific gene on the X chromosome that is passed down in a typical X-linked inheritance pattern. *Retinitis pigmentosa* - This is a group of **inherited eye diseases** that can be inherited in **multiple patterns**, including autosomal dominant, autosomal recessive, and X-linked recessive. - While some forms (~10-20%) are X-linked, it is **not exclusively or primarily** an X-linked disorder, and other options (like X-linked color vision deficiency) are more **characteristically** known for their X-linked inheritance.

Q: Menkes disease is associated with which enzyme deficiency?

Copper-transporting ATPase deficiency. ***Copper-transporting ATPase deficiency*** - **Menkes disease** (also known as Menkes kinky hair syndrome) is an **X-linked recessive disorder** caused by mutations in the **ATP7A gene**, which encodes the **copper-transporting P-type ATPase**. - This deficiency leads to impaired intestinal absorption and cellular transport of copper, resulting in **copper deficiency** in various tissues despite normal or even elevated levels in the intestinal lumen and some other cell types. *Tyrosinase deficiency* - **Tyrosinase** is an enzyme involved in the synthesis of **melanin**, and its deficiency is characteristic of certain forms of **albinism**, not Menkes disease. - Albinism is primarily characterized by hypopigmentation of skin, hair, and eyes due to reduced melanin production. *Ceruloplasmin deficiency* - **Ceruloplasmin** is a copper-carrying protein, and its deficiency is a hallmark of **Wilson's disease**, another disorder of copper metabolism, but it results from mutations in the ATP7B gene (which encodes a different copper-transporting ATPase). - Wilson's disease leads to copper accumulation in tissues like the liver and brain, contrasting with the copper deficiency seen in Menkes disease. *Dopamine beta-hydroxylase deficiency* - **Dopamine beta-hydroxylase** is an enzyme responsible for converting **dopamine to norepinephrine**. - Its deficiency results in an inability to synthesize norepinephrine and epinephrine, leading to symptoms like **orthostatic hypotension** and ptosis, unrelated to copper metabolism.

Q: In Wilson’s disease, which of the following substances is excreted in lower amounts in the urine?

Methyl- Histidine. **Important Note:** In Wilson's disease with Fanconi syndrome (proximal renal tubular dysfunction), most substances show **increased** urinary excretion, not decreased. This question appears to test knowledge of what is NOT characteristically elevated. ***Methyl-Histidine*** (Most appropriate answer) - **Methyl-histidine** (3-methylhistidine) is a marker of **muscle protein breakdown** and is not directly affected by the renal tubular dysfunction in Wilson's disease - Unlike phosphate, amino acids, and glucose which are pathologically increased in urine due to Fanconi syndrome, methyl-histidine excretion remains **normal** or is unrelated to copper-induced renal damage - This represents the substance least affected by Wilson's disease pathology *Phosphorus* (Actually INCREASED, not decreased) - **Medically incorrect as stated**: Phosphorus (phosphate) is actually **INCREASED** in urine in Wilson's disease, not decreased - Fanconi syndrome causes **renal phosphate wasting**, leading to hyperphosphaturia and resultant hypophosphatemia - This is a characteristic feature of proximal tubular dysfunction *Serine* (INCREASED due to aminoaciduria) - **Serine** and other amino acids show **generalized aminoaciduria** in Wilson's disease due to impaired proximal tubular reabsorption - Urinary serine is **elevated**, not decreased *Phosphotyrosine* - A phosphorylated amino acid involved in cell signaling, not routinely measured clinically - Not characteristically implicated in Wilson's disease urinary patterns

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

Flavin-containing monooxygenase 3 (FMO3). ***Flavin-containing monooxygenase 3 (FMO3)*** - **Trimethylaminuria**, or "fish odor syndrome," is caused by a genetic defect in the **FMO3 enzyme**. - This enzyme is responsible for converting **trimethylamine (TMA)**, a breakdown product of certain foods, into an odorless form, **trimethylamine N-oxide (TMAO)**. *Fumarylacetoacetate hydrolase* - Deficiency in this enzyme causes **Tyrosinemia type 1**, a metabolic disorder affecting tyrosine metabolism. - It leads to severe liver disease, kidney dysfunction, and neurological crises, not a fishy body odor. *Methane monooxygenase* - This enzyme is found in certain **bacteria** (methanotrophs) and is involved in the metabolism of methane. - It is not present in humans and has no known link to human metabolic disorders like fish odor syndrome. *D-amino acid oxidase* - This enzyme is involved in the metabolism of **D-amino acids**, which are less common in mammals but found in bacteria and some foods. - Its deficiency is not associated with trimethylaminuria or a fishy body odor.

Q: Which of the following is a marker for neural tube defects?

↑Acetylcholinesterase. ***↑Acetylcholinesterase*** - Elevated **acetylcholinesterase (AChE)** in amniotic fluid is a highly specific marker for **open neural tube defects (NTDs)**. - AChE is normally confined to nervous tissue; its presence in amniotic fluid suggests leakage from exposed fetal neural tissue. - AChE provides higher **specificity** than AFP for confirming open NTDs. *↑Alkaline phosphatase* - Elevated **alkaline phosphatase (ALP)** in amniotic fluid is associated with abnormalities like **omphalocele** and **gastroschisis**, but not specifically NTDs. - ALP levels can also be elevated in other conditions and are not a primary marker for neural tube defects. *↑Pseudocholinesterase* - **Pseudocholinesterase (butyrylcholinesterase)** is found in the liver, plasma, and other tissues, and its elevation is not a specific marker for neural tube defects. - It is sometimes used as a marker for liver function or exposure to certain toxins, not for fetal malformations. *↑Alpha-fetoprotein (AFP)* - While **AFP** is elevated in maternal serum and amniotic fluid with open NTDs, it is a **screening marker** with lower specificity. - AFP elevation can occur in other conditions (abdominal wall defects, multiple gestation, fetal demise), making it less specific than acetylcholinesterase for confirming NTDs.

Q: Tuberous sclerosis caused by mutations in TSC2 gene encodes which of the following proteins?

Tuberin. ***Tuberin*** - The **TSC2 gene** codes for the protein **tuberin**, which forms a complex with hamartin (encoded by TSC1). - This **tuberin-hamartin complex** functions as a tumor suppressor by negatively regulating the mTOR pathway. *Hamartin* - **Hamartin** is encoded by the **TSC1 gene**, not TSC2. - While it forms a complex with tuberin (TSC2 gene product), it is a distinct protein. *Merlin* - **Merlin** is a protein encoded by the **NF2 gene**, which is associated with **Neurofibromatosis type 2 (NF2)**, not tuberous sclerosis. - It plays a role in cell growth regulation and cell-cell adhesion. *Ankyrin* - **Ankyrin** proteins are a family of adapter proteins that link integral membrane proteins to the spectrin-actin cytoskeleton. - They are involved in many cellular processes but are **not directly encoded by the TSC2 gene** nor are they primarily associated with tuberous sclerosis.

Q: Inheritance associated with congenital adrenal hyperplasia -

AR. ***AR*** - Congenital adrenal hyperplasia (CAH) typically results from deficiencies in enzymes involved in adrenal steroid synthesis, most commonly **21-hydroxylase deficiency**. - These enzyme deficiencies are inherited in an **autosomal recessive (AR)** pattern, meaning an individual must inherit two copies of the defective gene (one from each parent) to manifest the condition. *AD* - **Autosomal dominant (AD)** inheritance means only one copy of a defective gene is needed for the condition to appear. This is not the typical inheritance pattern for CAH. - While some rare forms of adrenal disorders can be AD, the classic presentation of CAH is not. *XR* - **X-linked recessive (XR)** inheritance refers to conditions caused by genes on the X chromosome, primarily affecting males. CAH does not follow this pattern. - Males are generally more severely affected than females in XR disorders, which is not the case for common forms of CAH. *XD* - **X-linked dominant (XD)** inheritance means a single copy of a defective gene on the X chromosome can cause the disorder. This pattern is not characteristic of CAH. - XD disorders often have different presentations in males and females, and CAH does not fit this genetic model.

Question 1

A 45-year-old man presents with severe abdominal pain and neurological symptoms. His urine turns dark red upon standing. Which enzyme deficiency is most likely responsible for his condition?

Accepted Answer

Porphobilinogen deaminase

Answer

ALA dehydratase

Answer

Ferrochelatase

Answer

Coproporphyrinogen oxidase

Question 2

Which of the following statements is true regarding Fragile X syndrome?

Accepted Answer

Fragile X syndrome is caused by a trinucleotide CGG repeat expansion in the FMR1 gene.

Answer

Males typically have an IQ between 20-40

Answer

10-20% of female carriers may have intellectual disabilities

Answer

Gain of function mutation in the FMR1 gene

Question 3

Lesch-Nyhan syndrome is caused by deficiency of which enzyme?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Answer

Xanthine oxidase

Answer

Adenine phosphoribosyltransferase (APRT)

Answer

Adenosine deaminase (ADA)

Question 4

Which disorder is most characteristically linked to the X chromosome?

Accepted Answer

X-linked color vision deficiency

Answer

Beta-thalassemia

Answer

Klinefelter syndrome

Answer

Retinitis pigmentosa

Question 5

Menkes disease is associated with which enzyme deficiency?

Accepted Answer

Copper-transporting ATPase deficiency

Answer

Tyrosinase deficiency

Answer

Ceruloplasmin deficiency

Answer

Dopamine beta-hydroxylase deficiency

Question 6

In Wilson’s disease, which of the following substances is excreted in lower amounts in the urine?

Accepted Answer

Methyl- Histidine

Answer

Phosphotyrosine

Answer

Phosphorus

Answer

Serine

Question 7

Fish odor syndrome is caused by deficiency of which enzyme?

Accepted Answer

Flavin-containing monooxygenase 3 (FMO3)

Answer

Fumarylacetoacetate hydrolase

Answer

Methane monooxygenase

Answer

D-amino acid oxidase

Question 8

Which of the following is a marker for neural tube defects?

Accepted Answer

↑Acetylcholinesterase

Answer

↑Pseudocholinesterase

Answer

↑Alpha-fetoprotein (AFP)

Answer

↑Alkaline phosphatase

Question 9

Tuberous sclerosis caused by mutations in TSC2 gene encodes which of the following proteins?

Accepted Answer

Tuberin

Answer

Hamartin

Answer

Merlin

Answer

Ankyrin

Question 10

Inheritance associated with congenital adrenal hyperplasia -

Accepted Answer

AR

Answer

AD

Answer

XR

Answer

XD

Genetic Disorders and Biochemical Pathology — MCQs

Genetic Disorders and Biochemical Pathology — MCQs

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