MRCP2-0003

A 23-year-old woman presents to the Emergency Department (ED) for evaluation. She reports experiencing rapid heartbeats that lasted for up to 20 minutes before returning to normal, and she felt dizzy and almost fainted during the episodes. This has happened four times in the past three months, with each episode lasting for about two to three minutes. She is a long-distance runner who exercises for up to two hours a day and drinks three cups of coffee daily.
During the examination, her blood pressure (BP) is 100/65 mmHg, and her pulse is 55 beats per minute (bpm) and regular. There are no murmurs, and her chest is clear.
The following investigations are conducted:
Investigations Results Normal Values
Haemoglobin (Hb) 140 g/l 120–160 g/l
White cell count (WCC) 6.2 × 109/l 4.0–11.0 × 109/l
Platelets (PLT) 180 × 109/l 150–400 × 109/l
Sodium (Na+) 138 mmol/l 135–145 mmol/l
Potassium (K+) 4.2 mmol/l 3.5–5.0 mmol/l
Creatinine (Cr) 80 µmol/l 50–120 µmol/l
Thyroid-stimulating hormone (TSH) 3.5 µU/l 0.17–3.2 µU/l
Glucose 4.8 mmol/l 3.9–7.1 mmol/l
An electrocardiogram (ECG) shows normal function, with a ventricular rate of 58 bpm. A chest X-ray (CXR) is normal.
What is the most likely diagnosis for this patient?