**Core Concept**
Skeletal fluorosis is a condition caused by excessive intake of fluoride, leading to bone damage and skeletal deformities. It occurs when the body absorbs more fluoride than it can safely excrete, resulting in the accumulation of fluoride in bone tissue.

**Why the Correct Answer is Right**
The development of skeletal fluorosis is directly related to the amount and duration of fluoride exposure. Ingestion of high levels of fluoride, typically above 20-30 mg/L in drinking water, can lead to skeletal fluorosis. The fluoride ions accumulate in bone tissue, causing an increase in bone density and leading to a range of symptoms including joint pain, bone fractures, and skeletal deformities. This occurs due to the inhibition of bone mineralization and the promotion of abnormal calcification of bone tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely too low, as even moderate levels of fluoride intake can cause skeletal fluorosis in some individuals. While low levels of fluoride are generally considered safe, high levels can still cause harm.
**Option B:** This option is likely too high, as the exact threshold for skeletal fluorosis development can vary depending on individual factors such as age, sex, and overall health. However, a commonly cited threshold is around 20-30 mg/L.
**Option C:** This option is likely incorrect, as the development of skeletal fluorosis is not directly related to the duration of fluoride exposure alone. While prolonged exposure can increase the risk, even short-term exposure to high levels of fluoride can cause significant harm.

**Clinical Pearl / High-Yield Fact**
The Centers for Disease Control and Prevention (CDC) recommend a maximum fluoride concentration of 4 mg/L in community water systems to prevent dental fluorosis and skeletal fluorosis.

**Correct Answer:** B. 20-30 mg/L