Genu valgum

A three-year-old boy is brought to the clinic due to gait disturbance. The patient’s parents have noticed that their son has been falling more frequently while running. In addition, they have observed that he appears to have “knock knees.” They deny any significant injury, and the boy has not been complaining of pain. He was born full-term via an uncomplicated vaginal delivery and is up to date on vaccinations. He can name familiar objects, climbs well, and follows instructions. On physical examination he is well-developed, well-nourished, and speaks in 3-4 word sentences. Physical examination is notable for bilateral genu valgum. His knees are nontender and he has full range of motion without pain. Which of the following is the most likely etiology of this patient’s condition?