Urea Cycle — MCQs
Ammonia in brain is trapped by
Hyperammonaemia inhibits the TCA cycle by depleting which of the following?
Ammonia formed in the brain is converted into
Ammonia is detoxified in brain to :
Which will activate carbamoyl phosphate synthase I?
Most common enzyme deficiency in the urea cycle is:
Which of the following enzymes is not involved in the urea cycle?
Which enzyme deficiency is responsible for Hyperammonemia type-1?
Transamination of Alanine results in formation of ?
A 2-month-old infant presents with vomiting, lethargy, and metabolic acidosis. Blood tests reveal elevated levels of ammonia. What is the most likely diagnosis?
Urea Cycle Indian Medical PG Practice Questions and MCQs
Question 1: Ammonia in brain is trapped by
- A. Alanine
- B. Aspartate
- C. Glutamine (Correct Answer)
- D. Ornithine
Explanation: ***Glutamine*** - Ammonia in the brain is primarily detoxified by its conversion to **glutamine** through the enzyme **glutamine synthetase**. - This reaction combines **ammonia** with **glutamate**, effectively trapping the toxic ammonia in a non-toxic form that can be transported out of the brain. *Alanine* - **Alanine** plays a role in ammonia transport within the **glucose-alanine cycle** between muscle and liver, but it is not the primary mechanism for trapping ammonia in the brain. - While it can be formed from pyruvate and glutamate, its formation is not the main brain ammonia detoxification pathway. *Aspartate* - **Aspartate** is involved in the urea cycle and as a neurotransmitter, but it does not directly trap free ammonia in the brain. - It participates in transamination reactions with alpha-ketoglutarate, forming oxaloacetate and glutamate, but this isn't the main ammonia trapping mechanism. *Ornithine* - **Ornithine** is a key intermediate in the **urea cycle**, which primarily occurs in the liver for the detoxification of ammonia. - It is not directly involved in trapping ammonia within the brain tissue itself.
Question 2: Hyperammonaemia inhibits the TCA cycle by depleting which of the following?
- A. succinate
- B. α-ketoglutarate (Correct Answer)
- C. malate
- D. fumarate
Explanation: ***a keto glutarate*** - **Hyperammonemia** leads to the depletion of **α-ketoglutarate** through its amination to form **glutamate** by glutamate dehydrogenase and subsequently glutamine by glutamine synthetase. - The removal of **α-ketoglutarate** from the TCA cycle impairs its ability to produce energy and essential intermediates, contributing to neurological dysfunction in hyperammonemia. *succinate* - **Succinate** is an intermediate in the TCA cycle, but its depletion is not the primary mechanism by which hyperammonemia inhibits the cycle. - The direct consumption of **α-ketoglutarate** for ammonia detoxification is the more direct and significant impact. *malate* - **Malate** is another intermediate in the TCA cycle but is downstream from **α-ketoglutarate**. - Its depletion is a consequence of overall TCA cycle inhibition, not the initial cause mediated by hyperammonemia. *fumarate* - **Fumarate** is also a TCA cycle intermediate and is produced after succinate. - Its levels would be affected by the overall inhibition of the cycle, but it is not the direct target or substrate for ammonia detoxification that depletes the cycle.
Question 3: Ammonia formed in the brain is converted into
- A. Glycine
- B. Urea
- C. Cysteine
- D. Glutamine (Correct Answer)
Explanation: ***Glutamine*** - **Ammonia** is detoxified in the brain by combining with **glutamate** to form **glutamine** via the enzyme **glutamine synthetase**. - This conversion is crucial because **glutamine** is non-toxic and can be safely transported out of the brain to the liver for further processing. *Glycine* - **Glycine** is an amino acid that can function as a neurotransmitter, but it is not the primary product of ammonia detoxification in the brain. - While it can be synthesized in the brain, it does not serve as the molecule to which toxic ammonia is directly converted for transport. *Urea* - **Urea** is the primary end-product of ammonia detoxification in the **liver** through the **urea cycle**. - The brain lacks the complete set of enzymes required for the **urea cycle**, so it cannot convert ammonia into urea. *Cysteine* - **Cysteine** is a sulfur-containing amino acid involved in protein synthesis and antioxidant defense, but it is not directly involved in the detoxification pathway of ammonia in the brain. - Its synthesis and metabolism are distinct from the process of ammonia sequestration.
Question 4: Ammonia is detoxified in brain to :
- A. Urea
- B. GABA
- C. Glutamine (Correct Answer)
- D. Uric acid
Explanation: ***Glutamine*** - In the brain, **ammonia** is primarily detoxified through its conversion into **glutamine** by the enzyme **glutamine synthetase**. - This process is crucial for preventing **neurotoxicity** as ammonia can disrupt neuronal function and energy metabolism. *Urea* - **Urea** is the primary end product of **ammonia detoxification** in the **liver** through the **urea cycle**. - While urea can cross the blood-brain barrier, it is not the main mechanism for local ammonia detoxification within brain cells. *GABA* - **GABA (gamma-aminobutyric acid)** is an **inhibitory neurotransmitter** formed from **glutamate**. - It plays a vital role in neuronal signaling but is not directly involved in the detoxification of ammonia in the brain. *Uric acid* - **Uric acid** is the end product of **purine metabolism** and acts as an antioxidant. - It is not directly involved in the detoxification pathway of ammonia in the brain or any other organ.
Question 5: Which will activate carbamoyl phosphate synthase I?
- A. N-acetyl glutamate (Correct Answer)
- B. ATP
- C. Acetyl-CoA
- D. Ornithine
Explanation: **N-acetyl glutamate** - **N-acetyl glutamate** is an **allosteric activator** of **carbamoyl phosphate synthase I (CPS I)**, which is the mitochondrial enzyme that catalyzes the first committed step of the **urea cycle**. - Its synthesis is stimulated by high levels of **arginine**, linking nitrogen load to urea production. *Acetyl-CoA* - Acetyl-CoA is a common **substrate** and **product** in various metabolic pathways, but it is not a direct activator of CPS I. - It is a precursor for the synthesis of **N-acetyl glutamate**, but does not activate CPS I directly. *Ornithine* - **Ornithine** is a key intermediate of the **urea cycle**, but it does not directly activate CPS I. - It combines with carbamoyl phosphate (the product of CPS I) in the second step of the urea cycle to form citrulline. *ATP* - **ATP** is a **substrate** used by CPS I to provide energy for the synthesis of carbamoyl phosphate. - While essential for the reaction, ATP itself does not act as an allosteric activator of the enzyme.
Question 6: Most common enzyme deficiency in the urea cycle is:
- A. Ornithine transcarbamoylase (OTC) deficiency (Correct Answer)
- B. Arginase deficiency
- C. Carbamoyl phosphate synthase I deficiency
- D. Argininosuccinate synthetase deficiency
Explanation: ***Ornithine transcarbamoylase (OTC)*** - **OTC deficiency** is the most common and often the most severe inherited disorder of the **urea cycle**, leading to a buildup of ammonia. - It is an **X-linked recessive** disorder, predominantly affecting males, though carrier females can also exhibit symptoms. *Arginase deficiency* - This deficiency affects the final step of the urea cycle, leading to the accumulation of **arginine** and its precursors. - It is less common than OTC deficiency and typically presents with a later onset and milder symptoms. *Carbamoyl phosphate synthase I deficiency* - **CPS I deficiency** is a severe form of urea cycle disorder but is less common than OTC deficiency. - It results in the inability to synthesize **carbamoyl phosphate**, a crucial substrate for the urea cycle, leading to severe hyperammonemia. *Argininosuccinate synthetase deficiency* - This deficiency, also known as **citrullinemia type I**, leads to the accumulation of **citrulline** in the blood. - While it is a significant urea cycle disorder, it is not as frequently encountered as OTC deficiency.
Question 7: Which of the following enzymes is not involved in the urea cycle?
- A. Arginase
- B. Argininosuccinate lyase
- C. CPS-II (Correct Answer)
- D. CPS-I
Explanation: ***CPS-II*** - Carbamoyl phosphate synthetase II is involved in **pyrimidine synthesis**, not the urea cycle. - It uses **glutamine** as a nitrogen donor and is located in the **cytosol**. *CPS-I* - Carbamoyl phosphate synthetase I is the **rate-limiting enzyme** of the urea cycle. - It catalyzes the formation of **carbamoyl phosphate** from **ammonia**, CO2, and ATP in the mitochondria. *Arginase* - Arginase is the **final enzyme** in the urea cycle, converting **arginine** to **ornithine** and **urea**. - This reaction occurs in the cytosol and releases urea for excretion. *Argininosuccinate lyase* - Argininosuccinate lyase catalyzes the cleavage of **argininosuccinate** into **fumarate** and **arginine**. - This is a key step in regenerating arginine for the final step of the urea cycle.
Question 8: Which enzyme deficiency is responsible for Hyperammonemia type-1?
- A. Arginase deficiency
- B. Arginosuccinate lyase deficiency
- C. Arginosuccinate synthase deficiency
- D. Carbamoyl phosphate synthetase I (CPS-1) deficiency (Correct Answer)
Explanation: ***Carbamoyl phosphate synthetase I (CPS-1) deficiency*** - This enzyme deficiency is classified as **Hyperammonemia type-1**, or **CPS1 deficiency**, and results in the inability to initiate the urea cycle. - **CPS-1** catalyzes the first committed step of the urea cycle, combining ammonia and bicarbonate to form carbamoyl phosphate. *Arginase deficiency* - This deficiency causes **Hyperargininemia**, which is a disorder of the urea cycle distinct from Hyperammonemia type-1. - Arginase is involved in the final step of the urea cycle, converting arginine to urea and ornithine. *Arginosuccinate lyase deficiency* - This deficiency leads to **Argininosuccinic aciduria**, another urea cycle disorder. - **Arginosuccinate lyase** is responsible for breaking down argininosuccinate into arginine and fumarate. *Arginosuccinate synthase deficiency* - This deficiency causes **Citrullinemia type 1**, a metabolic disorder characterized by high levels of citrulline and ammonia. - **Arginosuccinate synthase** catalyzes the condensation of citrulline and aspartate to form argininosuccinate.
Question 9: Transamination of Alanine results in formation of ?
- A. Oxaloacetate
- B. Pyruvate (Correct Answer)
- C. Aspartate
- D. Arginine
Explanation: **Pyruvate** ✓ - **Transamination** involves the transfer of an amino group from an amino acid to an α-ketoglutarate (catalyzed by aminotransferases). - When **alanine** undergoes transamination via **ALT (alanine aminotransferase)**, its amino group is transferred to α-ketoglutarate, forming glutamate, while alanine is converted to its corresponding α-keto acid, which is **pyruvate**. - Reaction: Alanine + α-Ketoglutarate ⇄ Pyruvate + Glutamate *Oxaloacetate* - **Oxaloacetate** is the α-keto acid formed from the transamination of **aspartate** (via AST/GOT). - It is a key intermediate in the **citric acid cycle** and gluconeogenesis, not a product of alanine transamination. *Aspartate* - **Aspartate** is an amino acid, not an α-keto acid. - It can be formed from oxaloacetate via transamination (reverse reaction), and is involved in the **urea cycle** and nucleotide synthesis. *Arginine* - **Arginine** is a semi-essential amino acid, not an α-keto acid or a product of alanine transamination. - It plays roles in **protein synthesis**, the urea cycle, and nitric oxide production.
Question 10: A 2-month-old infant presents with vomiting, lethargy, and metabolic acidosis. Blood tests reveal elevated levels of ammonia. What is the most likely diagnosis?
- A. Urea Cycle Disorder (Correct Answer)
- B. Phenylketonuria
- C. Galactosemia
- D. Maple Syrup Urine Disease
Explanation: ***Urea Cycle Disorder*** - The combination of **vomiting**, **lethargy**, metabolic acidosis, and **elevated ammonia** in a 2-month-old infant is highly indicative of a urea cycle disorder due to the impaired detoxification of ammonia. - **Ammonia is neurotoxic**, explaining the lethargy, and its accumulation leads to severe metabolic derangements. *Phenylketonuria* - Characterized by the inability to metabolize **phenylalanine**, leading to its accumulation and neurological damage. - While it can cause developmental delay and seizures, it typically does **not present with acute metabolic acidosis** or hyperammonemia in infancy. *Galactosemia* - An inherited disorder of **galactose metabolism** that can cause vomiting, lethargy, and liver dysfunction, but is typically associated with **jaundice**, hepatomegaly, and **reducing substances in urine**, not primarily hyperammonemia and metabolic acidosis as the presenting features. - The primary defect is in the conversion of galactose to glucose, not ammonia detoxification. *Maple Syrup Urine Disease* - A rare metabolic disorder caused by a defect in the metabolism of **branched-chain amino acids** (leucine, isoleucine, valine). - Presents with feeding difficulties, lethargy, and a characteristic **sweet-smelling urine** ("maple syrup" odor), which is not mentioned in this case.
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