A thin 16 year old female cross country runner presents to student health because she hasn't had a menstrual period in several months. She had menarche at around 14, but her periods have been irregular. She fervently denies any possibility of being pregnant; she is not sexually active. She has no significant past medical history. On review of systems, she notes feeling cold and dizzy at times. She drinks a great deal of caffeine but eats very little. She is 163 cm (64 inches) tall and weighs 43 kg.
Challenge: Two legs of a triad have been described here. What's the third?