The metamorphosis of a tadpole into a frog requires the hormone thyroxine. If the water in which the tadpole lives lacks one specific mineral, thyroxine cannot be made and metamorphosis stops. That mineral is:
AIodine
BPhosphorus
CSodium
DCalcium
Answer & Solution
Correct answer: A. Iodine
Thyroxine is an iodine-containing hormone produced by the thyroid. Without enough iodine in the surrounding water, tadpoles cannot make thyroxine and therefore cannot complete metamorphosis. (In humans, iodine deficiency in childhood causes goitre and developmental problems — which is why iodised salt is widely promoted.)
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