From the given history: No fever Mass in right lumbar region Spine changes - Pyonephrosis, Appendicular lump in retrocecal position, Torsion of Right undescended testis can be ruled out as these conditions wont present with these clinical features Psoas abscess: Psoas Abscesses: The psoas muscle is another location in which abscesses are encountered. Psoas abscesses may arise from a hematogenous source, by contiguous spread from an intra- abdominal or pelvic process, or by contiguous spread from nearby bony structures (e.g., veebral bodies). Associated osteomyelitis due to spread from bone to muscle or from muscle to bone is common in psoas abscesses. When pott's disease was common, Mycobacterium tuberculosis was a frequent cause of psoas abscess. Currently, either S. aureus or a mixture of enteric organisms including aerobic and anaerobic gram-negative bacilli is usually isolated from psoas abscesses in the United States. S. aureus is most likely to be isolated when a psoas abscess arises from hematogenous spread or a contiguous focus of osteomyelitis; a mixed enteric flora is the most likely etiology when the abscess has an intra- abdominal or pelvic source. Patients with psoas abscesses frequently present with fever, lower abdominal or back pain, or pain referred to the hip or knee. CT is the most useful diagnostic technique."- Harrison 19/e p852 "Spinal TB (Pott's disease or tuberculous spondylitis) often involves two or more adjacent veebral bodies. Whereas the upper thoracic spine is the most common site of spinal TB in children, the lower thoracic and upper lumbar veebrae are usually affected in adults. From the anterior superior or inferior angle of the veebral body, the lesion slowly reaches the adjacent body, later affecting the interveebral disk. With advanced disease, collapse of veebral bodies results in kyphosis (gibbus). A paraveebral "cold" abscess may also form. In the upper spine, this abscess may track to and penetrate the chest wall, presenting as a soft tissue mass; in the lower spine, it may reach the inguinal ligaments or present as a psoas abscess. "Harrison 19/e p1110