A 16-year-old girl presents to her GP with weight loss, falls, and poor co-ordination. She reports an eight-month history of unsteadiness of gait and more recently developed slurred speech and upper limb tremors. She denies any changes in bowel habits but mentions increased urinary frequency. She was referred to a cardiologist for recurrent chest pain and palpitations and is awaiting an appointment. On examination, she has scanning speech and gaze-evoked horizontal nystagmus. Bilateral intention tremors with marked past pointing are observed in her upper limbs, and there is pyramidal weakness of both legs with absent knee and ankle reflexes and extensor plantar responses. She has a high steppage gait and positive Romberg’s test. Investigations reveal an axonal sensory motor neuropathy, left ventricular hypertrophy, and elevated fasting plasma glucose. Given these findings, what is the likely diagnosis?