This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System Pa 1, Subject name: Physiology
Maximum amount of smooth muscle, in relation to wall thickness, is present in: 
Select and explain. 
A. Trachea, B. Bronchi, C. Terminal bronchioles, D. Respiratory bronchioles
C: Terminal bronchioles
Answer: C: Terminal bronchioles 
Explanation: The trachea and upper respiratory passages (first 7 generations) have cailage & glands in their walls but little smooth muscle. They are lined by ciliated epithelium. Intermediate-sized and small-sized bronchi and bronchioles do not contain cailage. Cilia are present till respiratory bronchioles Terminal bronchioles have maximum amount of smooth muscle.