Diseases or infections which are transmitted through sexual intercourse with infected persons are collectively called sexually transmitted diseases(STDs).
Examples- HIV leading AIDS, gonorrhoea, Syphilis, Chlamydiasis, Genital herpes, Genital warts, Hepatitis-B, Trichomonasis, Enterobiasis, Scabies, Candidiasis.
Myasthenia gravis is a chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles, which are responsible for breathing and moving parts of the body, including the arms and legs. In this condition, antibodies, which are proteins that normally attack foreign, harmful substances in the body, attack the neuromuscular junction.
So, the corrrect answer is 'Myasthenia gravis'.