You must do six month of internal medicine to sit for ABPN exam. It does not have to be six months of wards, for example, my schedule for my intership year:
ID
Neurology Ward
ER
Gen Surg
CCU
FP
Vasc Surg
PED
ICU
Med Ward
Rheumatology
OB/GYN
Med Ward
So, as you can see, ID,CCU,ICU, 2 blocks of med ward, and Rheum is 6 months of internal medicine. The rest of my blocks cover everything I need to qualify for AOA internship.
Talk to the intern director and they can help you out, as long as you get six months of internal medicine or medicine specialties, you are set