Deficiency of fluoride in drinking water and diet most commonly leads to which one of the following health problems in humans?

Introduction / Context:
Fluoride is a trace element that plays a significant role in dental health. In appropriate amounts, fluoride helps strengthen tooth enamel and makes it more resistant to acid attack by bacteria, thereby reducing the risk of dental caries. Both deficiency and excess of fluoride can cause characteristic conditions. Many public health and general science questions ask what happens when fluoride is lacking in the water supply. This question targets the health problem most commonly associated with fluoride deficiency.

Given Data / Assumptions:
• The focus is on deficiency of fluoride, not excess.• The options include tooth caries, mottling of tooth, bending of bones, stiffening of joints, and paralysis.• We assume typical environmental exposure patterns and standard textbook descriptions of fluoride related disorders.• We distinguish between conditions caused by low intake and by high intake of fluoride.

Concept / Approach:
Mild and adequate fluoride intake helps form fluorapatite in tooth enamel, which is more resistant to dissolution by acids produced by oral bacteria. When fluoride is deficient, enamel is relatively weaker and more susceptible to decay, leading to higher incidence of dental caries. In contrast, excessive fluoride intake, especially during tooth development, can cause dental fluorosis, which presents as mottling or brown discolouration of teeth. Bone deformities and joint stiffness are more associated with skeletal fluorosis due to long term excess intake. Therefore, the correct answer for deficiency is tooth caries.

Step-by-Step Solution:
Step 1: Identify that the question is specifically about deficiency of fluoride.Step 2: Recall that fluoride strengthens enamel and protects against dental caries.Step 3: Understand that low fluoride levels mean weaker enamel and greater risk of tooth decay.Step 4: Remember that mottling of teeth and skeletal changes are typically associated with excess fluoride, not deficiency.Step 5: Select tooth caries due to decreased resistance of enamel to decay as the most appropriate effect of fluoride deficiency.

Verification / Alternative check:
Public health guidelines for community water fluoridation are based on the observation that communities with moderate natural fluoride levels have lower rates of dental caries. When such communities shift to low fluoride sources, caries rates tend to rise. On the other hand, areas with very high fluoride levels show dental or skeletal fluorosis. These epidemiological patterns support the conclusion that inadequate fluoride intake is associated with increased tooth decay, while mottling is a sign of excessive exposure.

Why Other Options Are Wrong:
Mottling and brown staining of teeth due to fluorosis occur when fluoride intake is excessive, not when it is deficient. Bending and deformity of bones are more characteristic of conditions like rickets, which involve vitamin D and calcium deficiency, or skeletal fluorosis from high fluoride. Stiffening of joints can occur in advanced skeletal fluorosis or in many arthritic conditions, again usually due to excess fluoride or other causes. Immediate paralysis of facial muscles is not a recognised consequence of fluoride deficiency. These options do not match the specific effect of low fluoride mentioned in the stem.

Common Pitfalls:
A frequent source of confusion is mixing up the effects of fluoride deficiency with those of fluoride excess. Students may see options like mottling of tooth and associate them with fluoride without noticing that the question is about deficiency rather than toxicity. To avoid this, always read carefully whether the question refers to deficiency or excess. Remember the simple rule: low fluoride is linked to more dental caries, while high fluoride is linked to dental or skeletal fluorosis.

Final Answer:
Deficiency of fluoride commonly leads to Tooth caries due to decreased resistance of enamel to decay.