Thanks for the query
Pain in left side of abdomen associated with flexion deformity
of hip and knee may be due to problem in joint or muscles attached to joint. You have not mentioned, is your child running fever or he had suffered any injury. Cause may be septic arthritis
, psoas abscess
, referred pain
from inflammation of hip joint or any bone injury
So I would suggest you to consult an orthopedician immediately for proper diagnosis.
I hope this answers your query effectively.
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Wishing you and your baby good health.