Which of the following improves skeletal strength and growth but is harmful in excess?

The correct choice in this question highlights the potential dangers of lead as a substance that can improve skeletal strength but is harmful when present in excess. While lead does play a role in the development of bones, especially during childhood, high levels of lead exposure can lead to severe health consequences, including developmental issues and damage to the nervous system.

In contrast, calcium and vitamin D are essential nutrients that contribute to bone density and overall skeletal health. Calcium is a fundamental component of bone structure, and vitamin D improves calcium absorption, both crucial for healthy bones. However, these substances do not carry the same toxicity risks when consumed in moderation.

Iron is another vital mineral, important for various bodily functions, including oxygen transport in the blood, but it does not directly improve skeletal strength like calcium and vitamin D do. While iron's excess can lead to toxicity, it is not primarily associated with skeletal growth in the same context as lead.

Overall, the knowledge around lead serves as a crucial reminder of how certain substances, while potentially useful in small amounts for skeletal development, can become hazardous at higher levels.