Final answer:
Nutcracker syndrome is likely the correct diagnosis for a unilateral varicocele that does not empty when the patient is lying down, due to the compression of the left gonadal vein, a condition that differs from other options because it can change with the patient's position.
Step-by-step explanation:
A patient with a unilateral varicocele that fails to empty when lying down may be experiencing Nutcracker syndrome. This condition occurs when the left gonadal vein, which drains the left testis and forms part of the spermatic cord, is compressed between the superior mesenteric artery and the aorta. This is significant because the right testicular vein empties directly into the inferior vena cava, while the left testicular vein goes into the left renal vein. The compression in Nutcracker syndrome results in impeded blood flow leading to varicocele, which typically resolves when the individual is recumbent and thus relieves the compression. Other options like renal vein thrombosis, hydrocele, or spermatocele do not typically resolve or change positionally in this way.