Pathognomonic signs are specific symptoms or findings that are so characteristic of a particular disease that they can be used to definitively diagnose it. These signs are unique to the disease and are not found in any other condition, making them invaluable in clinical diagnosis.
Differential diagnosis is a systematic method used by healthcare providers to identify a disease or condition in a patient by considering and ruling out multiple possible causes based on the patient's symptoms and medical history. It is crucial for ensuring accurate diagnosis and effective treatment, as it involves comparing and contrasting similar conditions to arrive at the most probable diagnosis.