**Core Concept**
Sodium fluoride is used in the treatment of otosclerosis, a condition characterized by abnormal bone growth in the middle ear, leading to hearing loss. The correct use of sodium fluoride involves understanding its effects on bone metabolism and otosclerosis pathology.

**Why the Correct Answer is Right**
Sodium fluoride is believed to inhibit osteoclastic activity, not osteoblastic activity. Osteoclasts are responsible for bone resorption, whereas osteoblasts facilitate bone formation. In otosclerosis, excessive osteoblastic activity contributes to abnormal bone growth. Sodium fluoride's mechanism of action is thought to involve the inhibition of osteoclasts, thereby reducing bone resorption and promoting bone remodeling. This is crucial in the management of otosclerosis.

**Why Each Wrong Option is Incorrect**
**Option B:** Sodium fluoride is typically used in the active phase of otosclerosis, but its effectiveness is still debated. The Schwartz sign (a reddish-blue discoloration of the promontory) is often associated with otosclerosis, but sodium fluoride's use is not strictly dependent on the presence of this sign.
**Option C:** Sodium fluoride does not have direct proteolytic activity on bone enzymes. Its effects on bone metabolism are more related to its ability to inhibit osteoclastic activity rather than possessing proteolytic properties.
**Option D:** While chronic nephritis may require careful management of electrolytes and fluids, it is not a direct contraindication for sodium fluoride use in otosclerosis.

**Clinical Pearl / High-Yield Fact**
Sodium fluoride is often used in conjunction with other treatments for otosclerosis, such as surgical interventions. Its effectiveness can vary depending on the severity of otosclerosis and individual patient factors.

**Correct Answer:**
✓ Correct Answer: A. It inhibits osteoblastic activity