uk-drcog MCQ (SBA): Hypothalamic Amenorrhoea - Osteoporosis Risk

A 22-year-old woman presents to the GP with amenorrhoea for 5 months. She has a BMI of 16 kg/m² and exercises excessively. She denies pregnancy. Her blood tests show: FSH 1.8 IU/L, LH 0.9 IU/L, oestradiol 40 pmol/L (low), prolactin normal, and TFTs normal. What is the most important long-term complication she is at risk of?