What is the physiological mechanism that produces blurred vision in
conjunction with enforced pupil dilation in an eye exam?

when dilating drops are added to the eye they temporarily paralyze the
sphincter muscles in the iris (the colored part of your eye).  As the
pupil widens, more light is shining on the retina than typical because
your pupil cannot adjust the aperture size.

Mydriatics: the drug in an eyedrop that paralyzes the sphincter muscle
and cilliary body muscles.

the opposite: Adrenergics like Pilocarpine make your pupil smaller.

When the aperture is too wide you cannot focus as before because there
are too many light rays hitting the retina.  Dilation blurs the near
vision and increases the brightness in a normal eye.

Also of interest:

Dark eyes do not dilate as widely or as quickly as light eyes.

Older people may not dilate as easily as younger folks.

Dilation can last from a few hours to a few days depending on the sensitivity.

clear vision depends on many factors, which are lost under mydriasis(
dilatation of pupil ) , because of following  factors vision under
mydriasis will not be clear.
the factors include :
1. adaptation of light entering the eye: the function of pupil is
adaptation of light by constricting and dialting the size of the pupil
by contractions of sphincter pupille and dilator pupille muscle. due
to the effect of mydriatic drugs the function of shincter pupille
muscle is reduced/ paralysed  so there will not be optimum adaptation
of light. clarity of vision is directly proportionate to the
adaptation of light, which is not possible under mydriasis , so
clarity of vision is disturbed.
2. size of the pupil : there is a formation of ''blurr circle'' around
each object which we see, like a darker colored ring , which is
directly proportional to the size of the pupil.so in dialted pupils
the blurr circle around the objects will be more than smaller pupils.
this increased blurr circle disturbs the clarity of vision.
4. sperical abbrations: these spherical abberations reduce the clarity
of vision , which are directly proportioanl to the size of the pupil .
under mydriatic dilated pupils , the spherical abbrations are more so
, the clarity of vision will be less.