Hi there, I have a two year old son who has been otherwise well apart from an URTI, who for the past 3 or 4 days has been complaining of intermittent back and stomach ache, accompanied with leg buckling in his left leg, with repeated falls. I have examined his knee and hip joints and can see no abnormality. I am an RGN nurse practitioner, interested in the diff diagnoses?
Hello and welcome, Recent upper respiratory tract infection and non-weightbearing (leg buckling) are highly suggestive of transient synovitis of the hip. The pain may be referred to the lower back. It usually affects 3-10 years old boys but may affect much younger child Chat Doctor.  Diagnosis is made clinically. Blood tests and ultrasound may be used to rule out other conditions. The condition is self limited (takes 1 to 2 weeks) and only needs supportive care (NSAIDs may be used). I hope I have answered your question. Please, feel free to ask me more if there is anything else you need to know. Thank you, Malik Among MD