What is the turning point of the play The Merchant of Venice?

You might find a range of different answers in response to
this excellent question. However, for me, the definite turning point in the play comes
in Act IV scene i, in the famous court scene, when Shylock looks as if he is just about
to gain his much-desired vengeance, before he is stopped by Portia disguised as the
famous lawyer. Note what she says:

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Tarry a little; there is something
else.

This bond doth give thee here no jot of
blood;

The words expressly are "a pound of
flesh."

Take then thy bond, take thou they pound of
flesh;

But in the cutting it if thou dost
shed

One drop of Christian blood, thy lands and
goods

Are by the laws of Venice
confiscate

Unto the state of
Venice.

Note how, with this
one speech, the situation of Shylock has been transformed from being a victor to being
defeated and outcast. Up until this point, the action has steadily pointed towards this
final confrontation, and the seemingly hopeless situation of Antonio and Shylock's ever
more desperate desire to gain his revenge. This is the turning point, because it
represents the moment when Shylock becomes defeated and Antonio now has power over
him.