**Core Concept**
Fluoride toxicity occurs when excessive fluoride ions disrupt normal cellular and physiological processes. The threshold dose of fluoride that necessitates immediate emergency treatment and hospitalization is a critical concept in managing acute fluoride poisoning.

**Why the Correct Answer is Right**
The threshold dose of fluoride that requires immediate emergency treatment and hospitalization is typically considered to be 5-10 mg/kg body weight. At this dose, fluoride ions can cause severe symptoms, including nausea, vomiting, abdominal pain, and in severe cases, cardiac arrest and seizures. The mechanism of fluoride toxicity involves the inhibition of the enzyme enolase, which is crucial for energy production in cells. Elevated fluoride levels can also lead to hypocalcemia, which contributes to the development of severe symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the threshold dose of fluoride for treatment is not 1 mg/kg. At this dose, symptoms are generally mild and may not require emergency treatment.

**Option B:** This option is incorrect because the threshold dose of fluoride for treatment is not 20 mg/kg. At this dose, symptoms are severe and require immediate emergency treatment, but the exact dose is typically considered to be 5-10 mg/kg.

**Option D:** This option is incorrect because the threshold dose of fluoride for treatment is not 15 mg/kg. While this dose may be close to the threshold, it is not the exact dose that requires immediate emergency treatment and hospitalization.

**Clinical Pearl / High-Yield Fact**
In cases of acute fluoride toxicity, it is essential to provide immediate treatment, including gastric lavage and supportive care, to prevent the progression of symptoms.

**Correct Answer:** C. 5-10 mg/kg body weight.