Four words have been given, out of which three are alike in some manner and one is different. Select the word that is different.

Psychiatrists

Option 2 , 3 , 4 studies the internal part of the body but option 1 studies IQ of the body

Cardiologists - to treat heart diseases

Nephrologists - to treat diseases related to kidney

Pulmonologists - to treat lungs and respiratotry organ

Psychiatrists - to treat the mental illness