Final answer:
Olfactory receptors rapidly undergo aging, which causes a drop in the intensity of a smell.
Step-by-step explanation:
The rapid drop in the intensity of a smell is caused by the aging of olfactory receptors. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. However, with age, the ability of these neurons to be replaced decreases, leading to a decline in the sense of smell.