Final answer:
The answer to the question which histological change does not occur as the respiratory tree branches is that cartilage does not increase; it decreases.
Step-by-step explanation:
The student has asked which of the following is not a change in histology as the respiratory tree branches from larger to smaller passageways. The choices were a) Submucosal glands disappear, b) Cartilage increases, c) Smooth muscle first increases, then decreases, and d) Goblet cells disappear. The correct answer is b) Cartilage increases. As the respiratory tree branches from larger bronchi to smaller bronchioles, the amount of cartilage actually decreases, eventually disappearing at the level of the bronchioles which are supported by elastic fibers instead. The other options correctly describe histological changes as submucosal glands and goblet cells decrease and smooth muscle increases then decreases.