D. Intersphincteric fistula with lower rectal/pelvic abscess: This is the most likely diagnosis because the presence of a rectal fistula without an external perianal opening, combined with a pelvic collection in the posterior aspect on CT, suggests an internal fistula that has led to an abscess formation. The intersphincteric plane is a common path for fistulas to track, and the absence of an external opening indicates that the fistula may have formed an internal collection or abscess.

A. Pelvi-rectal fistula: This could be a possible diagnosis given the pelvic collection, but it is less specific regarding the fistula's origin and path compared to an intersphincteric fistula with an associated abscess.
C. Ischiorectal abscess: While an ischiorectal abscess could explain the pelvic collection, it doesn't directly account for the fistula unless it's considered as a complication or an extension of the abscess.

Diverticulitis with fistula formation: Although less common in the differential for a rectal fistula, diverticulitis can lead to fistula formation and pelvic abscesses, especially if the diverticulitis is complicated. Missing this diagnosis could lead to significant morbidity if not properly managed.
Malignancy (e.g., rectal cancer) with fistula formation: A fistula and pelvic collection could be a presentation of advanced rectal cancer. This diagnosis is critical not to miss due to the significant implications for treatment and prognosis.

Crohn's disease with fistula formation: While Crohn's disease can cause fistulas and abscesses, it is less common in the rectum compared to other parts of the gastrointestinal tract. However, it remains a consideration, especially in patients with a known history of inflammatory bowel disease.
Traumatic or iatrogenic fistula: A fistula could be the result of trauma or a complication from a medical procedure. This would be less common but should be considered based on the patient's history.