Disorders of Urea Cycle — MCQs
Which enzyme in the Krebs cycle is indirectly affected by hyperammonemia due to its impact on metabolic pathways?
Which of the following statements about carbamoyl phosphate synthase is incorrect?
Which amino acid is used by the liver in the urea cycle?
A newborn presents with metabolic acidosis, high ammonia, and orotic acid in urine. Which enzyme deficiency is most likely?
Citrullinemia type I results from a deficiency of the
A patient presented to the emergency department with nausea and vomiting, and intravenous glucose was given, resulting in the patient's recovery. After a few months, the patient presented with the same complaints. Blood glutamine was found to be increased, and orotic acid levels were also raised. What is the diagnosis?
In alkaptonuria, deficiency is:
Which enzyme deficiency is responsible for Hyperammonemia type-1?
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?
Transamination of Alanine results in formation of ?
Disorders of Urea Cycle Indian Medical PG Practice Questions and MCQs
Question 1: Which enzyme in the Krebs cycle is indirectly affected by hyperammonemia due to its impact on metabolic pathways?
- A. Alpha-Ketoglutarate dehydrogenase (Correct Answer)
- B. Isocitrate dehydrogenase
- C. Succinate dehydrogenase
- D. Malate dehydrogenase
Explanation: ***Alpha-Ketoglutarate dehydrogenase*** - Hyperammonemia leads to the conversion of **alpha-ketoglutarate** into **glutamate** by glutamate dehydrogenase, which then uses ammonia to form **glutamine**. - This depletion of **alpha-ketoglutarate**, a substrate for alpha-ketoglutarate dehydrogenase, indirectly inhibits the enzyme's activity and thus the Krebs cycle. *Isocitrate dehydrogenase* - This enzyme is regulated by factors like **ATP**, **NADH**, and **ADP**, but not directly by ammonia or a substrate depletion caused by hyperammonemia. - Its activity is crucial for the cycle but not the primary or most direct target of ammonia's metabolic effects. *Succinate dehydrogenase* - This enzyme is part of both the **Krebs cycle** and the **electron transport chain**, but its activity is not directly or indirectly affected by ammonia detoxification pathways. - Its regulation is primarily linked to **FADH2** production and the electron transport chain. *Malate dehydrogenase* - This enzyme converts **malate** to **oxaloacetate** and is not directly impacted by the metabolic shunting of **alpha-ketoglutarate** due to hyperammonemia. - Its activity is critical for regenerating **oxaloacetate** to continue the cycle.
Question 2: Which of the following statements about carbamoyl phosphate synthase is incorrect?
- A. Requires biotin as a cofactor (Correct Answer)
- B. Enzyme found in the cytosol
- C. Enzyme found in mitochondria
- D. Catalyzes a condensation reaction
Explanation: ***Requires biotin as a cofactor*** - This is the **incorrect** statement and therefore the correct answer to this question. - Carbamoyl phosphate synthase (both CPS I and CPS II) does **NOT require biotin** as a cofactor. - Biotin is a cofactor for **carboxylase enzymes** such as pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, and methylcrotonyl-CoA carboxylase. - Carbamoyl phosphate synthase requires **ATP** and **Mg²⁺** but not biotin. *Enzyme found in mitochondria* - This statement is **correct**. - **Carbamoyl phosphate synthase I (CPS I)** is located in the **mitochondrial matrix** and catalyzes the first step of the urea cycle. - CPS I uses free ammonia (NH₃) as the nitrogen source and is activated by N-acetylglutamate. *Enzyme found in the cytosol* - This statement is **correct**. - **Carbamoyl phosphate synthase II (CPS II)** is located in the **cytosol** and is involved in de novo pyrimidine biosynthesis. - CPS II uses the amide nitrogen of glutamine (not free ammonia) as the nitrogen source. *Catalyzes a condensation reaction* - This statement is **correct**. - Both CPS I and CPS II catalyze the condensation of CO₂ (as bicarbonate), ammonia/glutamine, and two molecules of ATP to form carbamoyl phosphate, 2 ADP, and inorganic phosphate. - This is a complex reaction involving phosphorylation and condensation steps.
Question 3: Which amino acid is used by the liver in the urea cycle?
- A. Glutamine
- B. Glutamate
- C. Aspartate (Correct Answer)
- D. Ornithine
Explanation: ***Aspartate*** - **Aspartate** provides the second nitrogen atom to the urea cycle, directly contributing to the formation of **argininosuccinate** through condensation with citrulline. - It is crucial for the efficient removal of **ammonia** in the form of urea. *Glutamine* - **Glutamine** transports ammonia from peripheral tissues to the liver and kidneys, but it is typically deamidated to **glutamate** before its nitrogen can enter the urea cycle. - While it's a major ammonia carrier, it's not directly incorporated into urea as an intact amino acid. *Glutamate* - **Glutamate** can donate its amino group to form **aspartate** (via transamination with oxaloacetate) or release ammonia directly (via glutamate dehydrogenase), both of which then enter the urea cycle. - However, glutamate itself is not directly incorporated into the urea molecule in the same way aspartate is. *Ornithine* - **Ornithine** is an amino acid that participates in the urea cycle as a carrier molecule, being regenerated at the end of each cycle. - While essential for the cycle to function, it is not "used" in the sense of being consumed or providing nitrogen for urea formation - rather it acts as a catalytic intermediate that is recycled.
Question 4: A newborn presents with metabolic acidosis, high ammonia, and orotic acid in urine. Which enzyme deficiency is most likely?
- A. Arginase
- B. Glutamine synthetase
- C. Ornithine transcarbamylase (Correct Answer)
- D. CPS-1
Explanation: ***Ornithine transcarbamylase*** - Deficiency of **ornithine transcarbamylase (OTC)**, a key enzyme in the urea cycle, leads to the accumulation of **carbamoyl phosphate**. - **Carbamoyl phosphate** is then shunted to the pyrimidine synthesis pathway, leading to increased production and excretion of **orotic acid** in the urine, along with **hyperammonemia** and **metabolic acidosis**. *Arginase* - **Arginase deficiency** in the urea cycle primarily causes increased **arginine levels** and **hyperammonemia**, but it does not typically lead to the accumulation of orotic acid. - The effects are often more chronic with gradual onset of symptoms, rather than severe neonatal presentation with orotic aciduria. *Glutamine synthetase* - **Glutamine synthetase** catalyzes the synthesis of glutamine from glutamate and ammonia, playing a crucial role in **ammonia detoxification**. - A deficiency would impair ammonia detoxification and lead to **hyperammonemia**, but it would not directly cause **orotic aciduria**. *CPS-1* - **Carbamoyl phosphate synthetase I (CPS-1)** deficiency is the first step of the urea cycle and leads to severe **hyperammonemia** due to the inability to form carbamoyl phosphate. - Unlike OTC deficiency, **CPS-1 deficiency** does not involve the buildup of carbamoyl phosphate; hence, **orotic acid levels** would be low or normal, not high.
Question 5: Citrullinemia type I results from a deficiency of the
- A. Isocitrate dehydrogenase
- B. Argininosuccinate synthase (Correct Answer)
- C. Pyruvate dehydrogenase
- D. Succinyl CoA synthase
Explanation: ***Argininosuccinate synthase*** - **Citrullinemia type I** is an **autosomal recessive disorder** characterized by a deficiency of the enzyme **argininosuccinate synthase**. - This enzyme is crucial in the **urea cycle**, catalyzing the conversion of **citrulline** and **aspartate** into **argininosuccinate**. Its deficiency leads to the accumulation of citrulline and ammonia. *Isocitrate dehydrogenase* - **Isocitrate dehydrogenase** is an enzyme involved in the **citric acid cycle** (Krebs cycle) and plays a role in energy production, not the urea cycle. - A deficiency in this enzyme would affect cellular respiration and lead to different metabolic disorders, unrelated to ammonia detoxification. *Pyruvate dehydrogenase* - **Pyruvate dehydrogenase** is a multi-enzyme complex that converts **pyruvate** to **acetyl-CoA**, linking glycolysis to the citric acid cycle. - Deficiency in pyruvate dehydrogenase leads to lactic acidosis and neurological problems, distinct from the hyperammonemia seen in citrullinemia. *Succinyl CoA synthase* - **Succinyl CoA synthase** (or succinate thiokinase) is another enzyme of the **citric acid cycle** that catalyzes the reversible reaction of succinyl-CoA to succinate. - Its deficiency would impair energy metabolism within the mitochondria and would not directly lead to the accumulation of citrulline or hyperammonemia.
Question 6: A patient presented to the emergency department with nausea and vomiting, and intravenous glucose was given, resulting in the patient's recovery. After a few months, the patient presented with the same complaints. Blood glutamine was found to be increased, and orotic acid levels were also raised. What is the diagnosis?
- A. CPS-I deficiency
- B. Ornithine transcarbamoylase deficiency (Correct Answer)
- C. CPS-II deficiency
- D. Argininosuccinate synthetase deficiency
Explanation: ***Ornithine transcarbamoylase deficiency*** - **Ornithine transcarbamoylase (OTC) deficiency** leads to the accumulation of **carbamoyl phosphate**, which is shunted into pyrimidine synthesis, causing elevated **orotic acid** levels. - The elevated **glutamine** and the presentation of nausea and vomiting, responsive to glucose, suggest **hyperammonemia**, which is characteristic of OTC deficiency. *CPS-I deficiency* - **Carbamoyl phosphate synthetase I (CPS-I) deficiency** leads to a block in the first step of the urea cycle, causing **hyperammonemia**, but **without elevated orotic acid** because carbamoyl phosphate is not produced. - While patients would present with similar symptoms of **hyperammonemia**, the absence of **orotic aciduria** differentiates it from OTC deficiency. *CPS-II deficiency* - **Carbamoyl phosphate synthetase II (CPS-II)** is involved in **pyrimidine synthesis**, not the urea cycle. A deficiency here would not typically cause **hyperammonemia** or significant urea cycle dysfunction. - Instead, it would primarily affect pyrimidine production and might lead to megaloblastic anemia, which is not indicated here. *Argininosuccinate synthetase deficiency* - **Argininosuccinate synthetase deficiency** (citrullinemia type I) causes accumulation of **citrulline** and **hyperammonemia**. - This condition is characterized by very high plasma **citrulline** levels and does not typically present with elevated **orotic acid** alone as the primary distinguishing metabolic marker.
Question 7: In alkaptonuria, deficiency is:
- A. Phosphofructokinase
- B. HMG CoA reductase
- C. Homogentisate oxidase (Correct Answer)
- D. Xanthine oxidase
Explanation: ***Homogentisate oxidase*** - **Alkaptonuria** is an autosomal recessive disorder caused by a deficiency of the enzyme **homogentisate 1,2-dioxygenase** (also known as homogentisate oxidase). - This enzyme is crucial in the **catabolic pathway of tyrosine**, specifically breaking down **homogentisic acid**. *Phosphofructokinase* - Deficiency of **phosphofructokinase** (PFK) causes **Tarui's disease** (Glycogen Storage Disease Type VII), affecting **glycolysis**. - Symptoms include exercise intolerance, muscle pain, and hemolysis, which are unrelated to alkaptonuria. *HMG CoA reductase* - **HMG-CoA reductase** is the rate-limiting enzyme in **cholesterol biosynthesis**, and its inhibitors (statins) are used to lower cholesterol levels. - Its deficiency is not associated with alkaptonuria. *Xanthine oxidase* - **Xanthine oxidase** is involved in the catabolism of **purines**, converting hypoxanthine to xanthine and then xanthine to uric acid. - Its deficiency causes **xanthinuria**, leading to kidney stones, and it is not associated with alkaptonuria.
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: 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.
Question 10: 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.
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