Perirectal Abscess: This is the most likely diagnosis given the description of a multiloculated abscess in the perirectal region. The location and extension posteroinferiorly just proximal to the anus are consistent with the typical presentation of a perirectal abscess, which often results from an infection of the anal glands.

Perianal Abscess: While similar to a perirectal abscess, a perianal abscess is located closer to the anus and may not extend as far into the perirectal space. However, the distinction can sometimes be blurry, and clinical correlation is necessary.
Rectal Fistula or Sinus: These conditions can present with abscess formation and may have a similar appearance on imaging. A fistula or sinus could be the underlying cause of the abscess, especially if there's a history of recurrent infections.
Infected Rectal Cyst or Tumor: Though less common, a cyst or tumor in the rectal area could become infected and present as an abscess. This would be an important consideration if the patient has a history of rectal masses or if the abscess does not respond to typical treatment.

Necrotizing Fasciitis: This is a life-threatening condition that requires immediate intervention. While it might not be the first consideration for a multiloculated perirectal abscess, any signs of severe infection, such as significant pain out of proportion to examination, fever, or signs of systemic toxicity, should prompt consideration of necrotizing fasciitis.
Fournier's Gangrene: Similar to necrotizing fasciitis, Fournier's gangrene is a severe, life-threatening infection involving the genital and perineal area. It can extend to the perirectal region and requires urgent recognition and treatment.

Actinomycosis: A rare bacterial infection that can cause abscesses and fistulas in the abdominal and pelvic regions, including the perirectal area. It's often associated with the use of intrauterine devices (IUDs) or other foreign bodies.
Tuberculous Abscess: In areas where tuberculosis is common, a tuberculous abscess could be a rare cause of a perirectal abscess. This diagnosis would be considered in patients with risk factors for tuberculosis or those who do not respond to standard antibiotic therapy.