Firstly, as $\angle S P Q$ and $\angle S R Q$ are both
$90^{\circ}$, line $S Q$ must be the diameter of the circle with
midpoint being centre of circle, $M$.
As $A P R D$ is a rectangle, $\angle A P R$ and $\angle D R P$
are both right angles. Also lines $P M = R M$, as $P$ and $R$ are
on the circle.
Midpoint $M$ is also the centre of the rectangle. Let $V$ be
the length of the perpendicular from $M$ to side $A B$, and let $U$
be the length from the foot of this perpendicular to $P$, as shown
in the diagram.
$U = [y(x-y) + x(x+y)]/2 - A P$
$ = [x y - y^2 + x^2 + x y]/2 - y(x-y)$
$= [x^2 + y^2]/2$
$V = [y(x+y) + x(x-y)]/2$
$ = [x y + y^2 + x^2 -x y]/2$
$ = [x^2 + y^2]/2$
As $U = V$, $\angle B P M = 45^{\circ}$
$\Rightarrow \angle R P M = 45^{\circ}$
$\Rightarrow \angle P R M = 45^{\circ}$ (isosceles
triangle)
$\Rightarrow \angle P M R = 180^{\circ} - 45^{\circ} -
45^{\circ}= 90^{\circ}$
$\Rightarrow \angle P Q R = 45^{\circ}$ (angle at centre is
double that at edge - Circle Theorem)