Pilonidal sinus: This condition is characterized by a sinus tract in the skin, typically at the top of the cleft of the buttocks, which can become infected and discharge pus or caseous material. The increase in discharge upon pressure is consistent with the behavior of a pilonidal sinus, especially if it becomes infected.

Sacrococcygeal teratoma: Although less common, a sacrococcygeal teratoma can present as a mass in the lower back and, if it becomes infected, could potentially discharge caseous material. These tumors are congenital and can grow large enough to cause noticeable symptoms.
Postanal dermoid: A dermoid cyst located in the postanal region could potentially become infected and discharge material. However, this would be less likely to present with caseous discharge compared to a pilonidal sinus.

Tuberculous abscess: Although less common in some regions, tuberculosis can cause abscesses that discharge caseous material. Given the potential severity and the need for specific treatment, it's crucial not to miss this diagnosis.
Spinal dysraphism with infected dermal sinus: This is a congenital condition where there's an abnormal connection between the skin and the spinal canal. If infected, it could discharge material, including caseous material if the infection is due to certain pathogens.

Hydatid cyst: Caused by the Echinococcus parasite, hydatid cysts can form in various parts of the body, including the spine, and could potentially discharge caseous material if they become infected. However, this would be a rare presentation.
Chordoma: A rare type of bone cancer that can occur at the base of the spine. While it could present as a mass, discharge of caseous material would be highly unusual and suggests an advanced stage with infection.