In human physiology, which one of the following statements about the production of urea in the body is correct?

Introduction / Context:
Urea is the main nitrogenous waste product excreted by humans and many other mammals. Understanding where and how urea is produced is important for grasping the basics of excretion and liver function. This question checks whether you know the correct organ responsible for urea formation and can distinguish that from the organs that only help excrete or transport it.

Given Data / Assumptions:

• The focus is on the site of urea production in the human body.
• Options mention liver, blood, digestion of starch, and lungs and kidneys.
• Normal metabolism in a healthy adult is assumed, with functioning liver and kidneys.

Concept / Approach:
Proteins and amino acids are broken down in the body, releasing toxic ammonia. The liver converts this ammonia into urea through the urea cycle, also called the ornithine cycle. Urea is then released into the bloodstream, transported to the kidneys, and excreted in urine. It is not formed directly in the blood, nor is it a product of carbohydrate digestion or a primary product of lung or kidney metabolism. Therefore, the correct option must clearly identify the liver as the organ where urea is synthesized from nitrogenous wastes.

Step-by-Step Solution:
Step 1: Examine option A, which states that urea is produced in the liver from nitrogenous waste products. This matches the description of the urea cycle taking place in liver cells.Step 2: Examine option B, which says urea is produced in the blood. While urea is transported in blood, it is not synthesized there; blood is the medium, not the factory.Step 3: Examine option C, which claims urea is produced from starch digestion. Starch is a carbohydrate; its digestion produces simple sugars, not nitrogenous waste such as urea.Step 4: Examine option D, which attributes urea production to lungs and kidneys. Kidneys excrete urea but do not synthesize it; lungs primarily manage gas exchange.Step 5: Conclude that option A is the only accurate statement about urea production.

Verification / Alternative check:
To verify, recall that liver function tests often include measurements of blood urea and other liver related metabolites. In liver failure, ammonia levels rise because the urea cycle is impaired, confirming that the liver is essential for converting ammonia into urea. Also, patients with kidney failure accumulate urea in blood because the kidneys cannot excrete it, but its production in the liver may still continue.

Why Other Options Are Wrong:
Option B is wrong because blood only carries urea; it does not produce it. Option C is wrong because urea is a nitrogen containing compound derived from amino acids and proteins, not from carbohydrate digestion. Option D is wrong because kidneys filter urea from blood and lungs handle gases like oxygen and carbon dioxide; neither is the primary site of urea synthesis.

Common Pitfalls:
Students often confuse the place where a substance is produced with the place where it is excreted or transported. Another common mistake is to think that any organ associated with excretion, such as kidneys, must also produce the waste. Remember the sequence: proteins are broken down, the liver converts ammonia to urea, blood transports urea, and the kidneys excrete it. Keeping this pathway clear helps you avoid these errors in exams.

Final Answer:
Urea is produced in the liver from nitrogenous waste products.