Which chemical may cause methemoglobinemia, preventing red cell hemoglobin from transporting oxygen?

Methemoglobinemia occurs when hemoglobin is converted to methemoglobin, which is unable to bind oxygen effectively. Nitrates are known to cause this condition because they can be metabolized to nitrites in the body. The nitrites then oxidize the iron in hemoglobin from the ferrous (Fe2+) state to the ferric (Fe3+) state, forming methemoglobin. This conversion leads to a reduced ability of hemoglobin to carry oxygen, which can result in symptoms such as cyanosis, fatigue, and even more severe health issues if the levels are significantly elevated.

Although chloramines, fluoride, and aluminum can have various health effects, they are not predominantly associated with causing methemoglobinemia and do not share the specific mechanism through which nitrates exert this effect on hemoglobin. Thus, nitrates represent the most direct cause of the condition described in the question, making them the correct answer.