Step 1: Recall the disease.
Methemoglobinemia is also called blue baby syndrome. It mostly affects infants.
Step 2: Find the guilty ion.
Too much nitrate ion ($NO_3^-$) in drinking water causes it. So the answer is the nitrate ion.
Step 3: See what happens in the body.
Gut bacteria in babies change nitrate into nitrite: $NO_3^-\rightarrow NO_2^-$.
Step 4: See the effect on blood.
Nitrite changes the iron in haemoglobin from $Fe^{2+}$ to $Fe^{3+}$, making methaemoglobin.
Step 5: Understand the harm.
Methaemoglobin cannot carry oxygen well, so the baby gets too little oxygen and the skin turns bluish.
Step 6: State the answer.
\[ \boxed{NO_3^-} \]