what are some common causes of exercise-related injuries?
how can mental alertness to prevent injuries?
how can training specifically for a sport to prevent injury?

1. A “no pain, no gain” mentality, overtraining, and lack of knowledge about particular activities.

2. Studies have shown that a number of psychological factors might predispose an athlete to an acute injury. Different people have different “mind-sets” for the competition and intensity of exercise or sports. If you’re overly “psyched out” by the importance you place on the number of repetitions you can do or the number of games you win, or if the environmental conditions in which you must perform aren’t ideal (“it’s too windy/cold/hot”), your psychological state could detract from your performance and put you at risk for injury. If you’re already injured and are still engaging in activity, this, too, can be a distraction that puts you at greater risk.

3. Training specifically for a sport will allow a person to strengthen and develop the major muscle groups, movements, and energy systems that are unique to that particular sport.

a. cardiovascular system goes with d. atherosclerosis  

b. digestive system goes with a. cirrhosis  

c. endocrine system goes with b. addison's disease

d. immune system goes with c. mononucleosis

explanation:

don't worry, but do consult a doctor.

explanation:

having a high metabolism at age 13 is not abnormal.   however, if you are concerned, do consult a doctor.   the doctor may identify a correctable issue, and then you'll have your answer.   in the meantime, there are a few steps you might consider.

first, reflect on just how much physical activity (work out and dancing) you are getting relative to your caloric intake.   keep in mind that if you are interested in gaining weight, you must have a higher caloric intake than output.   second, if eating robust healthy meals is not providing enough calories, you'll need to include high caloric, yet healthy, snacks.   i warn against using an over the counter high-calorie supplement (drink or food) until you have consulted a doctor.