An infant is born in the breech position, and assessment indicates the presence of erb palsy (erb-duchenne paralysis). which clinical manifestation supports this conclusion?

The clinical manifestation which supports this conclusion is that an injury to the brachial plexus which happens during birth. Erb is termed as a paralysis of the arm which is caused by an injury on upper arm's main nerves to be specifically the C5-C6 nerves.
It arrives most commonly on shoulder dystocia when there is a difficult birth. This paralysis can resolve on its own depending on nature of the damage.