This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Most of the testosterone secreted by the testes exists in the plasma in the form of ? 
Select and explain. 
A. Dihydrotestosterone bound to gonadal steroid-binding hormone, B. Free dihydrotestosterone, C. Free testosterone, D. Testosterone bound to sex-steroid-binding globulin
D: Testosterone bound to sex-steroid-binding globulin
Answer: D: Testosterone bound to sex-steroid-binding globulin 
Explanation: The majority of circulating testosterone is bound to plasma protein (around 98%), rather than existing in free form . Of this, a majority is bound to a specific sex (or gonadal) steroid-binding protein , and a minority is bound to albumin. Dihydrotestosterone is produced from testosterone in the tissues by a specific enzyme, 5-alpha-reductase, rather than circulating in bound or free form. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 23. Function of the Male Reproductive System. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.