Excess of which ion in drinking water causes methemoglobinemia ?

Question

Accepted Answer

Methemoglobinemia, also known as "blue baby syndrome," is a condition where an abnormal amount of methemoglobin is produced in the blood. Methemoglobin is a form of hemoglobin that contains ferric [Fe3+] iron and has a decreased ability to bind oxygen, leading to reduced oxygen delivery to tissues.

1. **Explanation for Option 3 ($NO_{3}{}^{-}$):**
   - Nitrate ions ($NO_{3}{}^{-}$) in drinking water are known to cause methemoglobinemia, particularly in infants. When ingested, nitrates can be converted into nitrites ($NO_{2}{}^{-}$) in the body. Nitrites can oxidize the iron in hemoglobin from the ferrous (Fe2+) to the ferric (Fe3+) state, forming methemoglobin. This form of hemoglobin cannot effectively release oxygen to body tissues, leading to symptoms of oxygen deprivation.
   - Infants are particularly susceptible to this condition because their digestive systems are more likely to convert nitrates to nitrites, and their hemoglobin is more easily oxidized. This is why high levels of nitrates in drinking water are a significant health concern and are regulated by health authorities.

2. **Explanation for why other options are incorrect:**
   - **Option 1 ($F^{-}$):** Fluoride ions are not associated with methemoglobinemia. While excessive fluoride can lead to dental and skeletal fluorosis, it does not affect hemoglobin or oxygen transport in the blood.
   - **Option 2 ($SO_{4}{}^{2-}$):** Sulfate ions are not linked to methemoglobinemia. High levels of sulfate in drinking water can cause a laxative effect and gastrointestinal discomfort, but they do not interfere with hemoglobin function.
   - **Option 4 ($Cl^{-}$):** Chloride ions are a normal component of body fluids and are not associated with methemoglobinemia. Chloride is essential for maintaining fluid balance and acid-base balance in the body, and it does not affect the oxygen-carrying capacity of hemoglobin.

In conclusion, the presence of excess nitrate ions ($NO_{3}{}^{-}$) in drinking water is the specific cause of methemoglobinemia, making Option 3 the correct answer.