<s>[INST] My 15 yr old son noticed a lump (about 1/2 to 3/4 inch in size) which suddenly appeared on his lower left ankle. The lump seems to disappear at times, and is occasionally painful; it reappears randomly throughout the day, especially when seated. Had ultrasound today; saw a black circle which the tech took measurements of. What would appear so quickly? Cyst? Blood clot? [/INST] Hi, Dear,Welcome with your query to Chat Doctor. Studied your query in full depth of its details. Reviewed it in context of your health concerns. Based On the facts, You should go to your doctor as You mostly seem to suffer from-Varicose Veins lump above the ankle left-for your age without DVT-or blood Clot. This is not Cyst and Blood Clot either. Color Doppler Scan would fix this diagnosis-after Second Opinion from Surgeon. Cause could be leg Vein Varsity with Valve incompetency or by Chronic EXertional Stress Syn Chat Doctor. This reply would help you to plan further treatment soon with your treating doctors. Best of Luck and early recovery. Welcome any further query in this regard, which would be replied in next session. Good Day!! </s>