A 45-year-old female patient with diffuse osteoarticular pain, particularly low back pain, was referred by a rheumatologist for an updated radiologic evaluation. The patient had experienced these symptoms for many years and was diagnosed with human leukocyte antigen B27–negative spondyloarthritis approximately 11 years prior, based on findings of bilateral erosive sacroiliitis at pelvic radiography (Fig 1A) and bone scintigraphy with technetium 99m methylene diphosphonate (Fig 1B). After 3 years of treatment with a tumor necrosis factor–α inhibitor (adalimumab), which was effective for pain, the patient was lost to follow-up. At the current presentation, approximately 8 years after being lost to follow-up, the patient presented with worsening low back pain. The presence of nonobstructing kidney stones on US images confounded the underlying cause of worsening pain. The patient also experienced fatigue and depressed mood. Routine blood tests revealed a normal blood cell count, creatinine level of 0.64 mg/dL (56.58 μmol/L) (normal range, 0.30–1.1 mg/dL [26.52–97.24 mmol/L]), C-reactive protein level of 1.1 mg/dL (normal, <1 mg/dL), and vitamin D level of 21 ng/mL (52.42 nmol/L) (normal range, 30–100 ng/mL [74.88–249.60 nmol/L]). Noncontrast MRI of the thoracic and lumbar spine (Fig 2), MRI of the sacroiliac joints (Fig 3), and CT of the abdomen and pelvis (Fig 4) were performed.At the initial diagnosis, radiography of the pelvis (Fig 1A) showed slight widening of the sacroiliac joint, more visible on the left where a surrounding rim of sclerosis was also detected. Bone scintigraphy (Fig 1B) showed multiple symmetrical areas of radionuclide uptake in the sacroiliac joints, pelvic symphysis, and iliac crest, with less intense uptake noted bilaterally at the hips, shoulders, and ankles. At the current presentation, MRI of the thoracic and lumbar spine (Fig 2) showed diffuse, inhomogeneous, and moderately low signal intensity of the vertebral bone on T1-weighted images, suggesting changes in the bone matrix and replacement of normal fatty marrow. Furthermore, vertebral bodies had mild to moderate deformity with a biconcave shape due to bone remodeling, without cortical disruption. A multilocular expansile lesion associated with the right iliac bone that was not visible on the initial pelvic radiograph was shown on the sacroiliac MRI (Fig 3) and pelvic CT (Fig 4) scans. MRI characterized the pathologic composition of the lesion, showing inhomogeneous signal intensity and cystic patterning. The short τ inversion recovery sequence (Fig 3B) highlighted the difference between the hyperintensity of the fluid content and the hypointensity of the cyst wall and septations, which suggests hemosiderin deposits and fibrous tissue. There was no bone marrow edema surrounding the right iliac lesion. On CT images, the lytic expansile osseous lesion had a well-defined nonsclerotic margin and homogeneous internal fluid attenuation. Punctate areas of bony resorption were noted at the sacral margin of the sacroiliac joints. Integrated with the clinical data of nephrolithiasis and osteoarticular pain, imaging findings were interpreted in the spectrum of a metabolic or dystrophic disorder of bone tissue. With the differential consideration of primary hyperparathyroidism, further targeted blood examinations were requested, which showed a high level of parathyroid hormone (497 pg/mL; normal range, 18–88 pg/mL), hypercalcemia (calcium level, 12.6 mg/dL [3.15 mmol/L]; normal range, 8.3–10.6 mg/dL [2.08–2.65 mmol/L]), and a low serum phosphorus level (1.72 mg/dL [0.56 mmol/L]; normal range, 3.0–4.5 mg/dL [0.97–1.45 mmol/L]). The serum creatinine value was confirmed as normal (0.76 mg/dL [67.18 µmol/L]; normal range, 0.30–1.1 mg/dL [26.52–97.24 µmol/L]), as well as the 24-hour urinary calcium concentration (206 mg [5.15 mmol]; normal range, 100–300 mg [2.5–7.5 mmol]). The osseous alkaline phosphatase level was 75 U/L (1.25 µkat/L) (normal range, 35–104 U/L [0.58–1.74 µkat/L]) and thyroid thyrotropin level was 0.63 mIU/L (normal range, 0.4–4 mIU/L). The vitamin D level had increased from 21 to 35 ng/mL (52.42 to 87.36 nmol/L) (normal range, 30–100 ng/dL [74.88– 249.60 nmol/L]) as supplementation therapy had been started with cholecalciferol for presumed hypovitaminosis. Dual-energy x-ray absorptiometry indicated osteopenia, with T scores of −1.6 (lumbar spine) and −2.1 (left hip), both deviating from the normal reference range (+1 to −1) (1), and Z scores of −1.1 (lumbar spine) and −1.8 (left hip), also outside the accepted normal range of 2.5 to −2.5 (1), thus demonstrating below-average bone density. Parathyroid scintigraphy and SPECT/CT with 99mTc 2-methoxyisobutyl-isonitrile, or MIBI, (Fig 5) enabled confirmation of the diagnosis of primary hyperparathyroidism by revealing an area of avid tracer uptake adjacent to the superior pole of the left thyroid lobe, most consistent with parathyroid adenoma. The CT acquisition of the SPECT/ CT demonstrated rib cage deformities (Fig 5, arrows), another bony remodeling finding of primary hyperparathyroidism. The patient underwent parathyroidectomy and histologic assessment showed parathyroid adenoma. Biopsy of the lytic lesion, which was initially considered, was then deemed unnecessary. The patient started treatment with cholecalciferol and calcitriol. What is your diagnosis?