  We analyze the abstract representations of the groups of rational points of
even-dimensional quasi-split special unitary groups associated with quadratic
field extensions. We show that, under certain assumptions, such representations
have a standard description, as predicted by a conjecture of Borel and Tits.
Our method extends the approach introduced by the first author to study
abstract representations of Chevalley groups and is based on the construction
and analysis of a certain algebraic ring associated to a given abstract
representation.
