Hi Natasha,
Generally we know that sugars like to form 5 and 6 member rings, as these are the most themodynamically favored. Knowing more than this is beyond the scope of the MCAT.
As a result, the terminal CH2OH at the bottom generally does not attack to form sugars as this would result in a disfavored 7 member ring.
Please let me know if you'd like more clarification.
Katt