Differential Diagnosis for a Golf-Ball Size Sac of Fluid at the Base of the Penis

• Single Most Likely Diagnosis
  • Hydrocele: This is the most likely diagnosis, as a hydrocele is an accumulation of serous fluid in a body cavity, particularly in the tunica vaginalis testis or along the spermatic cord, which can present as a fluid-filled sac at the base of the penis.
• Other Likely Diagnoses
  • Epididymal cyst: These are fluid-filled cysts that can occur at the back of the testicle and might be felt at the base of the penis, especially if large enough.
  • Spermatocele: A spermatocele is a cystic structure that forms at the head of the epididymis and can contain sperm, presenting as a fluid-filled sac.
  • Varicocele: While typically presenting as a "bag of worms" due to dilated veins, a large varicocele could potentially be mistaken for a fluid sac.
• Do Not Miss Diagnoses
  • Testicular torsion: Although testicular torsion typically presents with acute pain and swelling of the testicle, any significant scrotal or penile swelling should prompt consideration of this emergency to prevent testicular loss.
  • Epididymitis: Inflammation of the epididymis can cause swelling and fluid accumulation, which might be mistaken for a sac of fluid, and requires prompt treatment to prevent complications.
• Rare Diagnoses
  • Lymphatic malformation or lymphocele: Abnormalities in lymphatic development can lead to fluid-filled sacs, though these are less common in the genital area.
  • Hematocele: Accumulation of blood in the scrotum, which could potentially be felt at the base of the penis if it extends or if there's significant bleeding.
  • Malignancy: Though rare, any unexplained mass or fluid accumulation should prompt consideration of malignancy, such as a testicular tumor or metastasis, until proven otherwise.