Human kidney stones are most commonly composed of which chemical substance?

Introduction / Context:
This question connects chemistry with human biology and health. Kidney stones are solid deposits that can form in the urinary system, and understanding their typical chemical composition is important for both medical science and general awareness. Many competitive exams ask which compound is the main constituent of common kidney stones.

Given Data / Assumptions:

• The focus is on human kidney stones.
• We must identify their most common chemical component.
• Options include calcium oxalate, sodium chloride, magnesium nitrate, calcium bicarbonate and ammonium sulphate.
• We assume typical dietary habits and ordinary clinical cases.

Concept / Approach:
Most kidney stones in humans are composed primarily of calcium compounds, and among these, calcium oxalate stones are the most common type. Less frequently, stones can be formed from uric acid, cystine or other salts. Sodium chloride is common salt and does not usually crystallise in the kidneys in this way. Magnesium nitrate and ammonium sulphate are not typical kidney stone constituents. Calcium bicarbonate is associated with temporary hardness of water but is not the standard composition of kidney stones.

Step-by-Step Solution:
Step 1: Recall from biology and health education that the majority of kidney stones are calcium based. Step 2: Among calcium salts, the most common in kidney stones is calcium oxalate, which can precipitate when urine is supersaturated with oxalate and calcium ions. Step 3: Examine option A, calcium oxalate. This matches the textbook statement that most kidney stones are calcium oxalate stones. Step 4: Option B, sodium chloride, is common salt that dissolves readily in water and does not typically form stones in the urinary tract. Step 5: Option C, magnesium nitrate, is a soluble salt that is not known as a major kidney stone component. Step 6: Option D, calcium bicarbonate, is associated with temporary hardness in water and can decompose to calcium carbonate, but this is not usually given as the main composition of kidney stones. Step 7: Option E, ammonium sulphate, is a fertiliser compound and not typical of kidney stones. Step 8: Therefore, the correct answer is calcium oxalate.

Verification / Alternative check:
Standard biology and general science textbooks explicitly mention that the majority of kidney stones are made of calcium oxalate crystals, sometimes mixed with calcium phosphate. Advice on preventing kidney stones often includes recommendations about reducing high oxalate foods such as spinach, nuts and certain chocolates in susceptible individuals, which again supports the idea that oxalate plays a key role. This consistent information across health sources confirms calcium oxalate as the correct choice.

Why Other Options Are Wrong:
Sodium chloride dissolves easily in urine and does not usually precipitate as stones. Magnesium nitrate is not a typical urinary salt and is not emphasised in medical literature on kidney stones. Calcium bicarbonate is more associated with water hardness and can convert to carbonate, but is not described as the main kidney stone material. Ammonium sulphate is used mainly as a fertiliser and does not match the usual composition of kidney stones. Hence, these options are incorrect for the specific question being asked.

Common Pitfalls:
Students may incorrectly choose calcium bicarbonate or think of calcium carbonate due to familiarity with chalk and water hardness. However, the specific phrase to remember for exams is calcium oxalate stones. Another pitfall is to get distracted by the presence of the word calcium in more than one option without recalling the role of oxalate in stone formation. Linking the term oxalate with common dietary advice for kidney stone patients can help solidify this association in memory.

Final Answer:
Human kidney stones are most commonly composed of calcium oxalate.