It was 5am on a typically busy night-shift in A&E and I'd been clerking non-stop for seven hours. Tired and grumpy, I picked up the next set of notes from the triage pile, which never seemed to grow any smaller.

Cynthia was complaining of fatigue and flu-like symptoms. As I examined her, my mind wandered to the queue of patients still waiting to be seen, and I longed for even just five minutes of respite to make a quick cup of tea or get a glass of water. Finding nothing of note, I discharged her home with some simple self-treatment advice.

The following evening, I opened my textbook. With my membership exam imminent, I was hoping to fit in a few hours of study before my next night-shift. 'Guillain-Barré syndrome,' the title read. Cynthia's image flashed into my mind. My stomach clenched. Oh god, how had I missed this?

I quickly scanned the page - the contents of which I already knew by heart. The fatigue, the weakness, the diarrhoea a few weeks previously - she had Guillain-Barré syndrome.

My heart raced. I read and re-read the page, hoping to find something that didn't fit, that would prove me wrong. But all I could see was the mother of three, who might now be critically ill or worse, thanks to my utter incompetence. She had a potentially fatal neurological condition, and I had sent her home with advice to stay in bed and take paracetamol.

Arriving at work, I searched frantically for the on-call consultant. 'You're right', he confirmed, listening to the panicked words tumbling from my mouth. 'She came back in this afternoon, she's in HDU.'

I felt like a lead weight had dropped into my stomach. My worst fear had been realised. I had missed an important diagnosis and my patient was now seriously ill. The guilt enveloped me, suffocating all other senses.

As the consultant re-read my clerking note, the perfectly documented Guillain-Barré history, I could feel myself crumbling with shame. What had I been thinking? My whole vision of myself as a competent doctor lay shattered in pieces on that emergency department floor.

Entering the HDU, there was Cynthia, sitting up in bed. 'She's doing fine,' the registrar told me. 'She'll be moving to the ward shortly'. The wave of relief was like none I had ever experienced.

The vice-like grip around my heart loosened and I could suddenly breathe again. I sat down at her bedside. 'I'm so sorry,' I managed, mortified to voice my failure. 'Don't worry,' she smiled, 'trust me to have something so rare'. Her words were generous, her kindness overwhelming.

The fear of making a mistake is a routine part of medical life. The gravity of dealing with human lives is so immense that this fear is not only natural, but important and instructive. It is the fear of getting it wrong that keeps me diligent and thorough, and is an inevitable consequence of caring about my patients.

Cynthia was my first medical mistake, one over which I self-flagellated for many months, but sadly it will not be my last. Despite our characteristic perfectionism, we doctors are only human, and mistakes are inescapable sequelae of our humanity. But such mistakes are not futile, and I will never forget the lessons I learned from Cynthia, or miss a diagnosis of Guillain-Barré syndrome again.