Could the thorn that was given to Paul be a mortal sin? In the 2 Cor 12:7-10 we read about Paul thorn:


  And to keep me from being too elated by the abundance of revelations,
  a thorn was given me in the flesh, a messenger of Satan, to harass me,
  to keep me from being too elated. Three times I besought the Lord
  about this, that it should leave me; but he said to me, “My grace is
  sufficient for you, for my power is made perfect in weakness.” I will
  all the more gladly boast of my weaknesses, that the power of Christ
  may rest upon me. For the sake of Christ, then, I am content with
  weaknesses, insults, hardships, persecutions, and calamities; for when
  I am weak, then I am strong.


Angelic Doctor commenting on mentioned paragraph says (emphasis added):


  In regard to the first it should be noted that very often a wise
  physician procures and permits a lesser disease to come over a person
  in order to cure or avoid a greater one. Thus, to cure a spasm he
  procures a fever. This the Apostle shows was done to him by the
  physician of souls, our Lord Jesus Christ. For Christ, as the supreme
  physician of souls, in order to cure greater sins, permits them to
  fall into lesser, and even mortal sins.


I am having difficulty in understanding how is it prudent to permit someone to fall into mortal sin to save him from the sin of pride and how is it possible that thorn (that was given to Paul) could be a mortal sin? 

Let me present my argument why it seems that the thorn that was given to Paul could not be mortal sin or more generally why it could not be held that God allows some to fall into mortal sin so to save them from the sin of pride:


A mortal sin involves putting some created thing as the final end of one's life.
That means that the human engaged in mortal sin distorts the order of himself in relation to God (ie. the human engaged in mortal sin does not put him in a proper position with respect of God).
Every disorder of one ordered in respect to God involves the sin of pride.
Therefore allowing one to commit mortal sin does not dimmish pride but rather increase it.


Also, it seems unfitting that God allows something so ugly as a mortal sin to prevent one from being prideful.  

Question: Could the thorn that was given to Paul be a mortal sin?
As a Protestant, let me offer a different interpretation: the phrase "thorn in the flesh" should not be taken in isolation - it has meaning in an Old Testament context. In Numbers 33:55 the Israelites were warned concerning the heathen Canaanites:
But if you do not drive out the inhabitants of the land, those you allow to remain will become barbs in your eyes and thorns in your sides. They will give you trouble in the land where you will live.

Secondly, Paul goes on to clarify: the "thorn" was a messenger of Satan. In other words, it was an evil spirit. Exactly how it afflicted him is not explained, though the idea that it stirred up the persecutions Paul suffered is one interpretation.