Final answer:
The adolescent's weight loss after starting methylphenidate therapy, commonly known as Ritalin, can be attributed to the medication's side effect of decreased appetite.
Step-by-step explanation:
The best explanation for the weight loss the adolescent experienced after beginning methylphenidate (Ritalin) therapy is most likely due to decreased appetite, which is a known pharmacological action of the drug. Methylphenidate is a stimulant medication often utilized in the treatment of ADHD. It increases neurotransmitter activity within the brain, notably in areas associated with impulse control. One of the therapeutic effects at the dosages prescribed for ADHD management includes decreased appetite.