A 46-year-old woman presents to her GP with a 2-month history of increasing tiredness and fatigue. She has also noticed that she has been getting more short of breath recently. Her past medical history includes two urinary tract infections in the past year and lower back pain for which she takes paracetamol. She does not take any other medications. On examination, she is pale. The GP orders some baseline blood tests:
Hb 101 g/ l(115–165 g/L)
MCV 88.1 fL (80-100 fL)
Platelets 129 * 109/l (140-400 * 109/l)
ESR 114 mm/h (3–9 mm/h)
WBC 3.2 * 109/l (4.0-11.0 * 109/l)
Na+ 137 mmol/l (135-145mmol/l)
K+ 4.9 mmol/l (3.5-5mmol/l)
Urea 10 mmol/l (2.5-6.7mmol/l)
Creatinine 108 µmol/l (45-90µmol/l)
eGFR 50 ml/min/1.73m2 (>90 ml/min/1.73m2)
Ca2+ 2.9 mmol/L (2.12-2.65mmol/L)
What is the next most appropriate investigation?
Renal ultrasound scan
Cervical lymph node biopsy
PTH levels
CT KUB
Serum electrophoresis