**Question:** A 65-year-old woman with type 2 diabetes is on hemodialysis for chronic kidney disease. She now presents to the clinic with symptoms of pain in the hands. The symptoms started many months ago and are now getting worse. She does not recall any injury to the hands and has not noticed any swelling or redness in the joints. On examination, the joints are normal with no inflammation or tenderness on palpation. There is full range of motion of the fingers and wrists. Lab investigations: calcium (7.2 mg/dL), phosphate (5.5 mg/dL), and PTH level (710 ng/L). (See Figure below) What is the most likely diagnosis?

A. Hypocalcemia
B. Hyperphosphatemia
C. Hyperparathyroidism
D. Kappa light chain deposition disease

**Core Concept:**

In this scenario, we are dealing with a patient on hemodialysis who presents with painful joints, specifically in the hands. The patient's symptoms have been slowly progressive and are associated with an underlying chronic kidney disease. The patient's lab investigations reveal elevated phosphorus (hyperphosphatemia) and parathyroid hormone (hyperparathyroidism) levels, along with normal calcium levels and a high intact PTH (710 ng/L).

**Why the Correct Answer is C:**

The correct answer, **C. Hyperparathyroidism**, is chosen due to the patient's history of chronic kidney disease, which is an important risk factor for developing secondary hyperparathyroidism. Secondary hyperparathyroidism is a common complication of chronic kidney disease and occurs due to the inability of the kidneys to adequately excrete the excessive parathyroid hormone, resulting in elevated levels. The elevated PTH levels lead to increased calcium absorption from the bones, leading to bone pain, joint pain, and muscle weakness, which are consistent with the patient's symptoms. The normal calcium and elevated intact PTH levels further support this diagnosis.

**Why Other Options are Incorrect:**

Option A (Hypocalcemia) is incorrect due to the normal calcium levels in the patient's blood. Hypocalcemia would result from low calcium levels, which is not observed in this case.

Option B (Hyperphosphatemia) is also incorrect as the patient's phosphorus levels are elevated, not hyperphosphatemia (elevated phosphorus levels).

Option D (Kappa light chain deposition disease) is an uncommon condition involving monoclonal gammopathies and is not directly associated with the patient's clinical presentation or laboratory findings.

In summary, the correct diagnosis of hyperparathyroidism in this case is supported by the combination of a patient's history, clinical presentation, and laboratory results. The key factors include the patient's chronic kidney disease leading to secondary hyperparathyroidism, elevated phosphorus levels, and normal calcium levels. The correct diagnosis is crucial for appropriate management, which may include medications, dietary modifications, and monitoring of PTH levels to manage the condition.