Final answer:
Mercutio is concerned about Tybalt's challenge because Tybalt is an expert swordsman and Romeo may not be ready for such a duel, especially as he is distracted by his love for Juliet, making the situation potentially deadly.
Step-by-step explanation:
In Act 2 of Romeo and Juliet, Mercutio is worried about Tybalt's challenge to the Montagues because Tybalt is well known for his skill with a sword. Mercutio knows that Romeo, blinded by his new love for Juliet, might not take the challenge seriously or be prepared to face Tybalt, who is an expert duelist. The seriousness of the challenge is heightened by the ongoing feud between the Montagues and the Capulets, which has already proven to be deadly. Hence, Mercutio fears that any confrontation with Tybalt could potentially lead to fatal consequences for his friend, Romeo.