Final answer:
Tetracyclines can cause permanent discoloration of teeth and skeletal defects if taken by children under 9.
Step-by-step explanation:
The class of medications that may cause permanent discoloration of teeth and skeletal defects if taken by children younger than 9 is tetracyclines. Tetracyclines are broad-spectrum antibiotics that can effectively treat a variety of bacterial infections, but they have certain side effects, such as phototoxicity, liver toxicity, and tooth discoloration. The permanent discoloration of developing teeth occurs when tetracyclines are used during tooth development, which is why they should not be given to children under the age of 9.