<s>[INST] My 15 year old son developed a hard, painless lump on the top of his right thigh. It is more prominent when he flexes his leg. It came out of nowhere over the weekend. He is in track at the moment and has been practicing, but states he did not injure it at anytime. What could this be? [/INST] Hello,The lump on thigh becoming more prominent with flexion indicates soft tissue lesion as lipoma or others. I recommend taking an ultrasound of the local part to diagnose it precisely.Hope I have answered your query. Let me know if I can assist you further.Regards, Dr. Bhagyesh V. Patel </s>