Urea cycle occurs in

Appeared in: KGMU 2024

Explanation

The urea cycle is a series of biochemical reactions that converts highly toxic ammonia into the less toxic compound urea.
This vital metabolic pathway is exclusively carried out in the liver cells (hepatocytes).
The cycle is compartmentalized, with the initial two steps occurring in the mitochondria and the subsequent steps in the cytoplasm of liver cells.
The produced urea is then released into the bloodstream, transported to the kidneys, and excreted in the urine.

Option A: The kidneys are responsible for filtering urea from the blood and excreting it in urine, but they do not synthesize urea through the urea cycle.
Option B: The brain is highly vulnerable to the toxic effects of ammonia. It has a protective mechanism to convert ammonia into glutamine, but it lacks the necessary enzymes for the complete urea cycle.
Option D: The lungs are primary organs for gas exchange (excreting carbon dioxide and taking in oxygen) and are not involved in nitrogenous waste metabolism.

Visual 1: Diagram: The Urea Cycle pathway, illustrating the five enzymatic steps and the involvement of both mitochondria and cytoplasm within a liver cell.
Visual 2: Flowchart: The journey of nitrogen from amino acid breakdown to urea excretion, highlighting the central role of the liver and the excretory function of the kidneys.

Clinical Relevance

Nursing practice connection: Knowing Location of the Urea Cycle helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
In severe liver disease (e.g., cirrhosis), the liver's ability to perform the urea cycle is impaired, leading to a buildup of toxic ammonia in the blood (hyperammonemia).
This hyperammonemia can cause a serious neurological condition called hepatic encephalopathy, characterized by confusion, lethargy, and coma. Nurses must monitor ammonia levels and mental status in patients with liver failure.
What if? If a newborn has a genetic deficiency in a urea cycle enzyme (an inborn error of metabolism), they would develop severe hyperammonemia shortly after birth, leading to brain damage if not treated immediately with a low-protein diet and medications to remove ammonia.

How to Approach the Question

This is a factual recall question from biochemistry.
First, identify the key concept in the question: 'Urea cycle'.
Recall the primary function of the urea cycle, which is the detoxification of ammonia from protein metabolism.
Associate this major metabolic and detoxification function with the primary metabolic organ of the body, the liver.
Evaluate the other options: Kidneys excrete urea, the brain is affected by ammonia but doesn't process it into urea, and the lungs are for respiration. This confirms the liver is the correct location.

Concept Tested & Keywords

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Practise the full KGMU 2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.