The reason for the bruising on the thighs includes the following aspects: firstly, it is considered to be caused by the abnormal coagulation function of the body, the lack of certain coagulation factors in the coagulation function, or the decrease in the number of platelets due to the dysfunction of the immune system of the body, which can cause the abnormal coagulation of the body, and then lead to the bruising on the thighs; secondly, it is considered to be caused by purpura, mostly allergic purpura, due to the exposure of the body to certain allergic substances. The second consideration is that purpura is caused by allergic purpura, due to the exposure of the body to certain allergic substances, which causes the local tiny blood vessels in the thigh to dilate and cause bleeding, resulting in bruising of the thigh. The reason for this is to go to the hematology department of the hospital and have blood tests such as routine blood tests, coagulation factors and blood sedimentation to identify the cause.