Final answer:
The first investigated case of child abuse involved Mary Ellen Wilson in 1873 which led to the development of child protection laws.
Step-by-step explanation:
The first case of child abuse was investigated in 1873 by Mary Ellen Wilson. She was a young girl living in New York whose case of abuse was brought to the attention of authorities, leading to legal intervention. Her situation became a milestone in the establishment of laws protecting children, as it prompted the creation of the New York Society for the Prevention of Cruelty to Children.