What is red line going up the thigh?

A red line going up the thigh can indicate a condition called lymphangitis, which is an inflammation of the lymph vessels often caused by an infection. It may appear as a streak of redness and can be accompanied by swelling, pain, and fever. This condition requires prompt medical attention, as it may indicate a spreading infection that could lead to more serious complications if left untreated. If you notice such a symptom, it's important to consult a healthcare professional.