7 yo boy has 6 day hx fever, pharyngitis, myalgias, difficulty walking. Last week: sore throat, body aches, fevers Tmax 103F. Fever decreased for last 3 days&he started eating normally. Started to walk strangely & walking on his toes when he got out of bed this morning. Several kids will ill last week. T 102.6F and tenderness over calves b/l & weakness on dorsiflexion. CK 2000.UA:no blood, LE, RBCs. Dx?