**Core Concept**
Decreased sense of smell, also known as anosmia, can be caused by various factors affecting the olfactory system, which includes the olfactory epithelium in the nasal cavity, the olfactory nerves, and the olfactory bulbs in the brain.

**Why the Correct Answer is Right**
The olfactory system is responsible for detecting odor molecules in the air, which bind to olfactory receptors on the surface of olfactory sensory neurons. The binding of odor molecules to olfactory receptors triggers a signal that is transmitted to the brain, where it is interpreted as a specific smell. Conditions that damage the olfactory epithelium, such as nasal polyps, or affect the transmission of signals from the olfactory epithelium to the brain, such as head trauma or neurodegenerative diseases, can lead to decreased sense of smell.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is actually associated with an increased sense of smell. Hyperosmia is a condition characterized by an abnormally acute sense of smell.
**Option B:** This option is incorrect because it is actually associated with an increased sense of smell. Certain neurological conditions, such as Parkinson's disease, can cause an increased sense of smell in some patients.
**Option C:** This option is incorrect because it is actually associated with decreased sense of smell. Rhinitis, an inflammatory condition of the nasal passages, can cause swelling and congestion that can lead to decreased sense of smell.

**Clinical Pearl / High-Yield Fact**
It's worth noting that decreased sense of smell can be an early symptom of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.

**Correct Answer: A. Hyperosmia**