1. Presented by Robert Kruse, MD, PhD, and reviewed by Sean Zhang, MD, PhD.

An adult with no significant past medical history presents with a 6-month history of an indolent, progressively worsening lower back pain complicated by profound weakness in bilateral lower extremities. This weakness has resulted in difficulty ambulating. Pain located to bilateral lumbar region and described as dull, aching, near constant. Pain worse with range of motion, standing from seated position. Neuro exam reveals a loss of sensation to light touch. The patient denies any headache, visual changes, hand clumsiness, nausea, vomiting, or bowel/bladder dysfunction. The patient also denies any bilateral upper extremity weakness, numbness, or paresthesia. This progressive weakness has been concurrent with a 30-lb unintentional weight loss and night sweats. No history of diabetes mellitus or hypertension. The patient denies intravenous drug use or epidural injections. The patient takes no daily medications. The patient is not immunocompromised. The patient has a history travel to the Southwest United States and the Middle East.

Due to intractable pain, the patient had an outpatient MRI without contrast, which demonstrated a large paraspinal soft tissue mass extending into the psoas muscle and adjacent vertebral body and disc space with associated pathologic compression fracture and severe canal stenosis. Imaging concerning for malignancy versus abscess/osteodiscitis. The patient was admitted for spinal canal stenosis with neurologic deficits. Interventional radiology biopsied the mass, which was found to be consistent with an abscess. Gram stain only revealed light polymorphonuclear cells, however. The patient was ultimately managed with a resection of mass and posterior vertebrectomy.

The excised tissue was submitted to pathology. GMS (Grocott's methenamine silver) and PAS (Periodic acid-Schiff) stains are shown below.

PAS stain


GMS stain

Question: What is the diagnosis?