Which of the following conditions in drinking water causes methemoglobinemia?

We begin by recalling the medical condition mentioned in the question, namely methemoglobinemia. Methemoglobinemia is a disorder in which the ferrous ion $$\text{Fe}^{2+}$$ present in normal hemoglobin is oxidised to the ferric state $$\text{Fe}^{3+}$$, producing methemoglobin, which is incapable of binding oxygen effectively. The result is impaired oxygen transport and, hence, cyanosis in infants. In the context of environmental chemistry, the most frequently cited cause of this ailment is an excessive concentration of nitrate ions $$\text{NO}_3^-$$ in drinking water.

The World Health Organization (WHO) guideline value and the Bureau of Indian Standards (BIS) permissible limit for nitrate in potable water are both $$50\ \text{mg L}^{-1}$$, which is the same as $$50\ \text{ppm}$$, because for dilute aqueous solutions $$1\ \text{ppm} \approx 1\ \text{mg L}^{-1}$$.

The biochemical sequence responsible for the disease is as follows. First, in the infant’s digestive system nitrate $$\text{NO}_3^-$$ is reduced to nitrite $$\text{NO}_2^-$$ through microbial action:

$$\text{NO}_3^- + 2\ \text{H}^+ + 2\ e^- \longrightarrow \text{NO}_2^- + \text{H}_2\text{O}$$

Next, nitrite oxidises the ferrous ion in hemoglobin:

$$\text{Hb(Fe}^{2+}\!) + \text{NO}_2^- \longrightarrow \text{MetHb(Fe}^{3+}\!) + \text{NO} + \text{OH}^-$$

Because methemoglobin (MetHb) cannot carry oxygen, its accumulation produces hypoxia symptoms in infants, a condition popularly called “blue baby syndrome.” Therefore, the critical threshold set by health authorities is $$\gt 50\ \text{ppm}$$ nitrate; water exceeding this value is unsafe for infants.

Let us compare this with each option given:

Option A states $$\gt 50\ \text{ppm}$$ of nitrate. We have just established that exceeding this concentration indeed leads to methemoglobinemia.

Option B cites chloride. Although high chloride imparts a salty taste and can corrode pipes, it is not associated with methemoglobinemia.

Option C refers to sulphate. High sulphate levels may cause a laxative effect but again are unrelated to the disease in question.

Option D points to lead. Lead contamination has serious neurological effects but does not produce methemoglobinemia.

Consequently, the only correct statement is that methemoglobinemia occurs when the nitrate level in drinking water exceeds $$50\ \text{ppm}$$.

Hence, the correct answer is Option A.