The red spots at the base of the thighs may be related to allergies, fungal infections, contact dermatitis and other reasons, not necessarily due to syphilis, it is recommended to consult a doctor to clarify the cause of early treatment. Local allergies, fungal infections, contact dermatitis and other causes can lead to red spots at the base of the thighs, mainly manifested as localized red spots accompanied by itching, pain, burning and other discomforts, and there is no abnormal syphilis serological test. Stage II syphilis may present with a syphilis rash manifested as red macules on the skin, without itching or slight itching. A positive syphilis spirochete test and antibody titer can help confirm the diagnosis. In summary, there are many reasons for red spots on the thighs, not necessarily due to syphilis, and it is recommended to seek medical attention to actively find the cause of the disease and treat the cause. If there is a history of high-risk sexual behavior or syphilis, it is recommended to undergo serologic screening for syphilis.