A 25-year-old man has a bluish lump in his groin that disappears when lying down. What is the most likely diagnosis?

The presence of a bluish lump in the groin that disappears when the patient lies down suggests the presence of a saphena varix. A saphena varix is a dilatation of the great saphenous vein near the saphenofemoral junction, typically associated with venous insufficiency. When the patient stands, the elevation of the limb can lead to pooling of blood and the formation of a visible lump due to increased venous pressure. However, upon lying down, the pressure decreases and the lump resolves, making this symptom characteristic.

This diagnosis is further supported by the age and general presentation of the patient, which fits the typical demographic for this condition. Inguinal hernias and hydroceles might present with a lump as well, but they usually do not exhibit a bluish coloration nor do they typically disappear simply by lying down.

Testicular torsion involves severe acute pain and does not cause a reducible lump in the groin; rather, it is associated with significant discomfort and potential testicular loss if not urgently addressed. Thus, the combination of the lump characteristics and its behavior when lying down strongly points to saphena varix as the most likely diagnosis.