Disorders of urea cycle

Disorders of urea cycle

Q. 1 Inherited hyperammonemia is a result of deficiency of which enzyme of Krebs-Henseleit urea cycle?

 A Malate dehydrogenase

 B

Isocitrate dehydrogenase

 C

N-acetyl glutamate synthetase

 D

Succinate dehydrogenase

Q. 1

Inherited hyperammonemia is a result of deficiency of which enzyme of Krebs-Henseleit urea cycle?

 A

Malate dehydrogenase

 B

Isocitrate dehydrogenase

 C

N-acetyl glutamate synthetase

 D

Succinate dehydrogenase

Ans. C

Explanation:

Inherited hyperammonemias are a group of six diseases caused by inborn deficiencies of the enzymes of the Krebs-Henseleit urea cycle.

The enzymes involved are:
1. N-acetyl glutamate synthetase
2. Arbamyl phosphate synthetase (CPS)
3. Ornithine transcarbamylase (OTC)
4. Argininosuccinic acid synthetase (citrullinemia)
5. Argininosuccinase deficiency
6. Arginase deficiency

Most Severe Cases: In the most severe forms of the hyperammonemic disorders, the infants are asymptomatic at birth and during the first day or two of life, after which they refuse their feedings, vomit, and rapidly become inactive and lethargic, soon lapsing into an irreversible coma. Profuse sweating, focal or generalized seizures, rigidity with opisthotonos, hypothermia, and hyperventilation have been observed in the course of the illness.

These symptoms constitute a medical emergency, but even with measures to reduce serum ammonia, the disease is usually fatal.

Ref: Ropper A.H., Samuels M.A. (2009). Chapter 37. Inherited Metabolic Diseases of the Nervous System. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor’s Principles of Neurology, 9e.


Q. 2

Citrullinemia is due to deficiency of ‑

 A

Argininosuccinate lyase

 B

Argininosuccinate synthase

 C

Arginase

 D

Ornithine transcarbamylase

Ans. B

Explanation:

Ans. is. B. Argininosuccinate synthase


Q. 3

In urea cycle, hydrolysis of arginine forms ‑

 A

Citrulline

 B

Ornithine

 C

Carbomoyl phosphate 

 D

Arginosuccinase

Ans. B

Explanation:

Ans. is ‘b’ i.e., Ornithine 


Q. 4

In urea cycle which defect is an X linkeddisease ‑

 A

Ornithine transcarbamylase

 B

Aspartate transcarbamylase

 C

Arginase

 D

Argininosuccinate synthase

Ans. A

Explanation:

Ans. is ‘a’ i.e., Ornithine transcarbamylase 



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