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
Which hormone act by crossing cell membrane - 
Select and explain. 
A. Thyroxine, B. Insulin, C. Glucagon, D. Calcitonin
A: Thyroxine
Answer: A: Thyroxine 
Explanation: Ans. A. ThyroxineIntracellular receptors are transcription factors that have binding sites for the hormone (ligand) and for DNA and function as ligand (hormone)-regulated transcription factors.Steroid hormones and the steroid derivative vitamin D3 fulfill this requirement.Thyroid hormones must be actively transpoed into the cell.