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.