Saudi Arabian IMO Booklet

Let $B{B}^{\prime }$, $C{C}^{\prime }$ be the altitudes of an acute-angled triangle $ABC$. Two circles passing through $A$ and $C^{\prime }$ are tangent to $BC$ at points $P$ and $Q$. Prove that $A$, $B^{\prime }$, $P$, $Q$ are concyclic.