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: Pharmacology
All of the following have receptors which are transcription factors, except: 
Select and explain. 
A. Insulin, B. Estrogen, C. Glucocoicoids, D. Vitamin D
A: Insulin
Answer: A: Insulin 
Explanation: Insulin receptors themselves are Enzymatic proteins and not Transcription factors. Ref: K D Tripathi Textbook of Pharmacology, 5th Edition, Pages 45, 214; Ganong Review of Medical Physiology By Barrett, Pages 320-21; Cell Physiology Source Book By Nicholas Sperelakis, Pages 657-58