A condition in which certain nerve cells are missing from the muscle layers of part of the large intestine. This causes severe constipation or blockage of the large intestine. Constipation is when stool becomes hard, dry, and difficult to pass and bowel movements occur less often than normal. Other symptoms include swollen abdomen, vomiting, bloody diarrhea, gas, lack of energy, and trouble gaining weight. Aganglionic megacolon is present from birth, but the symptoms may not appear until later in a child’s life. This condition has been linked to an increased risk of thyroid cancer and neuroblastoma. Also called Hirschsprung disease.