A 72-year-old man is brought to the Emergency Department by his daughter after several weeks of increasing fatigue, constipation, and generalised bone pain. She reports that he has also been more confused over the last 48 hours.

Observations:

• Heart rate: 92 beats per minute, regular
• Respiratory rate: 18 breaths per minute
• Blood pressure: 128/76 mmHg
• Temperature: 37.1 °C
• Oxygen saturation: 97% on room air
• GCS: 14 (E4, V4, M6)

Examination:

• Patient appears tired, mildly dehydrated, with dry mucous membranes
• Diffuse tenderness over the thoracic spine
• No focal neurological deficit
• Abdomen soft, non-tender
• No palpable lymphadenopathy

Past medical history: Hypertension, benign prostatic hypertrophy

Medication history: Ramipril, Tamsulosin

Social history: Retired builder, ex-smoker, minimal alcohol use

Investigations:

• Blood Glucose 4.1mmol/L
• ECG SR, rate 96, QTC 330ms

What is the most likely underlying cause of this patient's symptoms?