  In this article we classify all the smooth threefolds of P^5 with an apparent
quadruple point provided that the family of its 4-secant lines is an
irreducible (first order) congruence. This is sufficient to conclude the
classification of all the smooth codimension two varieties of P^n with one
apparent (n-1)-point and with irreducible family of (n-1)-secant lines.
