Final answer:
The most appropriate response is clarifying that despite the legality in some areas, marijuana has negative effects on the brain, heart, and lungs, especially when used during adolescence.
Step-by-step explanation:
In response to the statement from an adolescent client being treated for polysubstance abuse, the most appropriate response would be: "Evidence shows that marijuana has negative effects on the brain, heart, and lungs." While marijuana is legal in some jurisdictions, legality does not equate to a lack of harm. Long-term use, especially when started during adolescence, can impede brain development leading to difficulties with thinking, memory, and learning. Additionally, frequent smoking of marijuana can cause coughing and breathing problems. It's also important to note that, if a woman smokes marijuana during pregnancy, there can be problems with child development during and after pregnancy.