Final answer:
The extra chromosome causing Patient B's infertility is on pair number 21, which corresponds to Down Syndrome. Trisomy 21 is the most common trisomy among viable births.
Step-by-step explanation:
The extra chromosome that may be causing Patient B's infertility is on pair number 21.
This extra chromosome corresponds to Down Syndrome, a condition characterized by short stature, facial distinctions, and developmental delays. Trisomy 21, the presence of an extra chromosome on pair 21, is the most common trisomy among viable births.