In children, this disease is caused due to excess nitrate fertilizers usage

The correct option is methaemoglobinaemia.

What is Methaemoglobinaemia?
Methaemoglobinaemia, also commonly known as "blue baby syndrome" in infants, is a blood disorder in which an abnormal amount of methaemoglobin is produced. Methaemoglobin is a form of haemoglobin, the oxygen-carrying protein in red blood cells. Unlike normal haemoglobin, methaemoglobin cannot release oxygen effectively to body tissues.

How Excess Nitrate Fertilizers Cause This Disease:
1. Runoff and Contamination: When excess nitrate-based chemical fertilizers are used in agriculture, the surplus nitrates leach into the groundwater or run off into nearby water bodies, contaminating drinking water sources.
2. Conversion to Nitrites: When infants consume water or formula made with nitrate-contaminated water, bacteria in their digestive systems convert the nitrates (NO₃^-) into highly reactive nitrites (NO₂^-).
3. Interaction with Haemoglobin: Once absorbed into the bloodstream, nitrites oxidize the iron in normal haemoglobin from its divalent ferrous state (Fe²⁺) to its trivalent ferric state (Fe³⁺). This oxidized form is called methaemoglobin.
4. Oxygen Deprivation: Since methaemoglobin is unable to bind and transport oxygen, the body's tissues become deprived of oxygen. This leads to clinical symptoms such as cyanosis, where the baby's skin turns a bluish color due to lack of oxygen (hence the name "blue baby syndrome").