Small bowel carcinoid tumor or other neuroendocrine tumor: The presence of a large volume of soft tissue in the distal aspect of the small bowel mesentery, encasing and slightly effacing vessels, along with gross thickening of small bowel loops and upstream dilatation of fluid-filled small bowel loops, is highly suggestive of a carcinoid tumor or another type of neuroendocrine tumor. These tumors are known to cause desmoplastic reactions leading to fibrosis and potential bowel obstruction.

Lymphoma: Although there's no lymphadenopathy mentioned, lymphoma can cause masses in the mesentery and lead to bowel obstruction. The absence of lymphadenopathy does not rule out lymphoma, especially if it's an early or localized disease.
Metastatic disease: Metastases to the small bowel mesentery from other primary cancers (e.g., breast, lung, melanoma) could present similarly, causing obstruction. The patient's age and the presence of a significant abdominal mass make this a plausible consideration.
Sclerosing mesenteritis: This is a rare condition characterized by inflammation and fibrosis of the mesenteric fat, which can encase vessels and cause bowel obstruction. It might not be as likely given the specific findings, but it remains a possibility.

Mesenteric ischemia or infarction: Although the CT findings suggest a mechanical obstruction, it's crucial not to miss a diagnosis of mesenteric ischemia, which could be catastrophic if not promptly treated. The presence of dilated peripheral vessels and some free fluid could also be seen in ischemia.
Infectious or inflammatory processes: Conditions like mesenteric panniculitis or an infectious process could potentially cause similar symptoms and findings, especially if there's an element of bowel wall thickening and free fluid.

Desmoid tumor: A type of benign but locally aggressive tumor that can occur in the mesentery and cause bowel obstruction. These are rare and often associated with familial adenomatous polyposis (FAP) or Gardner syndrome.
Retroperitoneal fibrosis: Although more commonly associated with the retroperitoneum, this condition can extend into the mesentery and cause obstruction. It's characterized by fibrosis and inflammation in the retroperitoneum.
Intra-abdominal sarcoma: Sarcomas are rare tumors that can occur in the abdomen, including the mesentery, and could potentially cause the described findings.

Given the high suspicion for a mechanical obstruction due to a mass, the next best steps would include:

Surgical consultation: For potential surgical intervention to relieve the obstruction and obtain a tissue diagnosis.
Further imaging or diagnostic tests: Such as an MRI or a PET scan to better characterize the mass and assess for potential metastatic disease.
Supportive care: Including bowel rest, hydration, and possibly total parenteral nutrition (TPN) if the obstruction cannot be immediately relieved.
Tissue diagnosis: Through biopsy or surgical specimen to guide specific treatment (e.g., chemotherapy, targeted therapy) if a carcinoid tumor, lymphoma, or metastatic disease is confirmed.