Final answer:
The most common cause of a posterior mediastinal mass in children under 2 years old is neuroblastoma, a type of cancer arising from immature nerve cells, often located in the adrenal glands but can also appear in the abdomen, chest, or along the spinal cord.
Step-by-step explanation:
The most common cause of a posterior mediastinal mass in a child younger than 2 years old is neuroblastoma. Neuroblastoma is a cancer that develops from immature nerve cells found in several areas of the body and is most commonly found in the adrenal glands, which are located on top of the kidneys. It can also develop in other areas of the abdomen, chest, and in the nerve tissue that runs along the spinal cord in the back. While Wilms tumor affects the kidneys, lymphoma affects the lymphatic system, and teratoma is a germ cell tumor often found along the body's midline, neuroblastoma is specifically associated with nerve cells and is more likely to present as a mass in the posterior mediastinum, which is the area in the chest between the lungs.