Swollen glands is MY thing - I get it when I wear cheap earrings; I get it when I'm getting a cold; I get it when the pollen is heavy and thick; recently I got it before a really bad ear infection.... Everytime I'm sick, it either starts there or moves through there.....
If I were your sis I would NOT go back to that doctor, seems incompetent to me. Definitely get a second opinion., I still would not trust that the culture meant mono.
