Why Snape's worst memory?

[vmonte]

No: HPFGUIDX 137217

greg wrote:
So is this Snape's worst memory, not because of what we saw, but what
we were about to see? We know that Snape pulled Harry out of the
memory before it reached its conclusion. Is there something
additional that we're missing?

vmonte:
Several months ago this topic came up. Someone mentioned (I cannot 
remember who) that perhaps Snape figured out what Lupin was because 
of the essay question. I thought that the prank happened the night 
after the exam so here is our idea...

This memory leads into the night of the prank. In the penseive memory 
Harry sees Snape looking over his notes, while James and gang are 
joking about question ten: on how to spot a werewolf.  
This is when we think that Snape realizes what Lupin is (as if his 
name isn't a big clue). 

In OOTP Lupin mentions to Harry that Snape was always folowing them 
around and that he was very curious about Lupin's disapearance every 
month.

Anyway, While Harry is watching Snape's Worst Memory he notices that 
Lupin does not look good and wonders if the full moon is approaching.

I think that the prank happened that night after the exams and that 
Snape already knew what Lupin was.   

In PoA Snape gives this same essay to Lupin's class. And then 
Hermione figures out what Lupin is because of it.

Vivian