In a pediatric clinic, located in a rural area, there are 9 children, who simultaneously fell ill. The following signs were detected: low physical activity, acrocyanosis of the nasolabial triangle and fingertips, mucosal cyanosis, tachycardia, dyspnea. It was determined that all the sick - children were fed with a formula that was dissolved in the water taken from a dug well. Laboratory analysis revealed high levels of methemoglobin in the blood of the children. These signs can be caused by increased content of a certain element in the water. Name this element: