A 50-year-old man has felt vague abdominal discomfort

for the past 4 months. Physical examination does not reveal lymphadenopathy,

abdominal masses or organomegaly on palpation. Bowel sounds are heard. An

abdominal CT scan shows a 20 cm retroperitoneal soft tissue mass obscuring the

left psoas muscle. A stool specimen tested for occult blood is negative. Which

of the following neoplasms could it be?