**Core Concept**
Infantile methaemoglobinemia, a condition characterized by elevated levels of methaemoglobin, can be caused by excessive exposure to certain substances, including nitrates. Nitrates are metabolized to nitric oxide, which then binds to hemoglobin, forming methaemoglobin. This condition is particularly concerning in infants, who have lower levels of methaemoglobin reductase, an enzyme that converts methaemoglobin back to normal hemoglobin.

**Why the Correct Answer is Right**
The correct answer is related to the threshold for nitrate exposure that can lead to infantile methaemoglobinemia. Nitrates in excess of 10 mg/L can cause this condition, as they are metabolized to nitric oxide, which then binds to hemoglobin, forming methaemoglobin. This is particularly concerning in infants, who have lower levels of methaemoglobin reductase, making them more susceptible to this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is below the threshold for nitrate exposure that can cause infantile methaemoglobinemia. Nitrates below 10 mg/L are generally considered safe.
**Option B:** This option is incorrect because it is above the threshold for nitrate exposure that can cause infantile methaemoglobinemia, but it is not the correct answer. The correct answer is 10 mg/L.
**Option C:** This option is incorrect because it is not a valid concentration for nitrate exposure that can cause infantile methaemoglobinemia.

**Clinical Pearl / High-Yield Fact**
Infants are more susceptible to nitrate-induced methaemoglobinemia due to lower levels of methaemoglobin reductase. This condition can be prevented by ensuring that water and food products do not contain excessive nitrates.

**Correct Answer:** D. 10