I found nothing from 1850, 60 & 80 US Census on a John M. Alexander md to a Hardinor Drennan, though I have some data on several Drennan's/Drennon's of the area, including: 36MaleMulattoH D DrennonDistrict no 20 18, Wilson, Tennessee... From the 1860 US Slave Schedules; ergo, it appears that Hardinor's family were slave owners, as often times the slaves would take the surname of their owner(s).How this man would have been connected to your family & by whom ??. Also, there shows a David J. & Elvira Drennan living in Wilson County; David was born about 1824 and might be a brother of your 2nd great aunt. All J.M. and John & John M. Alexander's born in TN or living in TN from the 1860 to 1900 Census show no sign of your family, as most are farmers and none are doctors that I can find. Also, found nothing in findagrave.com.Sorry.