Final answer:
The principle of autonomy is violated when the patient is not told the truth about their medical diagnosis, preventing them from making informed decisions.
Step-by-step explanation:
The ethical principle that is violated when a patient is not told the truth about their medical diagnosis is the Principle of Autonomy. This principle states that patients have the right to exercise agency or self-determination when making decisions about their own healthcare. By withholding information from the patient, they are unable to make informed decisions about their course of treatment, which violates their autonomy. It is important for healthcare professionals to respect patient autonomy and provide them with all relevant information to make decisions that align with their values and preferences.