**Core Concept**
Crippling fluorosis is a severe form of skeletal fluorosis, a condition caused by excessive intake of fluoride over an extended period. It leads to bone deformities, joint pain, and crippling disability. The condition is dose-dependent and can be prevented by maintaining optimal fluoride levels in drinking water.

**Why the Correct Answer is Right**
The correct answer, **> 10 mg/litre**, is based on the World Health Organization's (WHO) guidelines for maximum allowable concentration of fluoride in drinking water. Intakes above this threshold can lead to crippling fluorosis, characterized by severe bone deformities and crippling disability. Chronic exposure to high fluoride levels can also cause dental fluorosis, skeletal fluorosis, and other health problems. The WHO recommends a maximum fluoride concentration of 1.5 mg/litre in drinking water to prevent these adverse effects.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.5-0.8 mg/litre is below the recommended level for optimal oral health and is unlikely to cause crippling fluorosis. In fact, this range is considered safe for most populations.
**Option B:** 1.5 mg/litre is the maximum recommended level for fluoride in drinking water, and intakes at this level are generally considered safe for most people.
**Option C:** 3-6 mg/litre is still below the threshold for crippling fluorosis, although it may increase the risk of dental fluorosis and skeletal fluorosis.

**Clinical Pearl / High-Yield Fact**
The optimal level of fluoride in drinking water for preventing dental caries is between 0.5-1.0 mg/litre, while levels above 1.5 mg/litre can increase the risk of crippling fluorosis.

**✓ Correct Answer: D. > 10 mg/litre. Excessive fluoride intake above this level can lead to crippling fluorosis, a severe form of skeletal fluorosis.**