A 27 year old man presents with symptoms characterized by alternating mood swings along with flight of ideas, elation, over activity and disinhibition, or low mood with lack of energy and social withdrawal. What is the most probable diagnosis?

The key is A. Bipolar affective disorder. [Flight of idea, elation, over activity and disinhibition are features of mania and low mood, lack of energy and social withdrawal are features of depression. Alternating mood swings with depression and mania are chracteristic of Bipolar disorder].