**Core Concept**
Intranasal steroids are a first-line treatment for nasal polyposis due to their anti-inflammatory properties. They work by inhibiting the production of inflammatory mediators such as histamine, leukotrienes, and cytokines, which contribute to the growth and maintenance of nasal polyps.

**Why the Correct Answer is Right**
Intranasal steroids are effective in reducing polyp size and symptoms in patients with nasal polyposis. They are also useful in preventing recurrence of polyps after surgical removal. The mechanism of action involves binding to glucocorticoid receptors in the nasal mucosa, which then regulates the expression of inflammatory genes.

**Why Each Wrong Option is Incorrect**
**Option A:** Intranasal steroids are not effective in treating nasal polyposis. This statement is incorrect as intranasal steroids have been shown to reduce polyp size and symptoms in patients with nasal polyposis.
**Option B:** Intranasal steroids have no systemic side effects. This statement is incorrect as systemic side effects such as glaucoma, cataracts, and growth suppression in children can occur with long-term use of intranasal steroids.
**Option C:** Intranasal steroids are not a first-line treatment for nasal polyposis. This statement is incorrect as intranasal steroids are considered a first-line treatment for nasal polyposis due to their efficacy and safety profile.

**Clinical Pearl / High-Yield Fact**
It's essential to use intranasal steroids with caution in patients with glaucoma, cataracts, or a history of eye surgery, as they can exacerbate these conditions.

**Correct Answer: C. Intranasal steroids are not a first-line treatment for nasal polyposis.