For negligence to have occurred, you have to establish that the doctor being sued had a duty of care towards the patient (a duty of care is owed towards anybody who you may reasonably foreseeably injure), that this duty of care was breached and this breach directly caused harm or loss to the patient.
This is commonly determined by the Bolam Test (1-) which basically says that if a body of experts in the same field as the doctor on trial determines that their actions were reasonable in that situation, then they are not guilty of negligence (yes, that’s a mouthful. Read it a few times if you didn’t get it). Therefore, a doctor (or any other healthcare professional) only needs to prove that a respected body of opinion found their actions to be acceptable practice. Judges have the authority to consider their opinion unreasonable if s/he personally finds it illogical, however in the majority of cases the fact that experts of a profession agree with the opinion will validate it.
If negligence is established, the case then has to prove causation, which is that had the negligence not occurred, the patient’s disease would have been diagnosed/ treated properly. For example, in one case (2-) a doctor was shown to be negligent by refusing to see a patient who went to A&E with stomach pains who then died, but evidence suggested that even if the doctor had seen the patient, there would have been nothing he could have done to save him and so the doctor was not found guilty as his actions did not cause the injury.
How to answer a question about negligence
It’s highly unlikely you’ll get a question about this but just incase you do, the most important thing here is the Bolam Test because it means that doctors are accountable to each other. This offers doctors a certain amount of protection, because often what may seem to others a major oversight, other doctors will understand to be simply par for the course. Doctors have a duty to care for their patients as they are trusting you with their health and, often, their lives- as a doctor it’s your duty to respect that. If you refer to the four principles, the key ones to respect here are beneficence and non-maleficence. You have to do what is best for your patient and not do anything to deliberately cause them harm.
1- Bolam v Friern HMC,  All ER 118,121
2- Barnett v Chelsea and Kensington HMC  1 All ER 1068