**Question:** Which one of the following features is NOT associated with 1° hyperparathyroidism?

A. Increased serum calcium levels
B. Increased serum phosphorus levels
C. Increased serum alkaline phosphatase levels
D. Vitamin D deficiency

**Core Concept:** Hyperparathyroidism is a condition where one of the parathyroid glands produces excessive amounts of parathyroid hormone (PTH). This leads to a cascade of physiological changes, including alterations in serum calcium, phosphorus, and alkaline phosphatase levels.

**Why the Correct Answer is Right:** Vitamin D deficiency is not typically associated with primary hyperparathyroidism. In primary hyperparathyroidism, there is an overproduction of PTH, which leads to increased calcium reabsorption in the kidneys and reduced calcium absorption in the intestines. As a result, serum calcium levels are typically elevated, but phosphorus and alkaline phosphatase levels are increased as well.

**Why Each Wrong Option is Incorrect:**

A. Increased serum calcium levels: This is a correct association with primary hyperparathyroidism, as mentioned above.
B. Increased serum phosphorus levels: Although serum phosphorus levels can be mildly elevated in some cases of primary hyperparathyroidism, it is less specific than elevated calcium levels as a diagnostic marker.
C. Increased serum alkaline phosphatase levels: Alkaline phosphatase is a marker for bone turnover, and its elevation in primary hyperparathyroidism indicates increased bone resorption due to increased bone remodeling.

**Clinical Pearl:** In clinical practice, primary hyperparathyroidism should be considered when presenting symptoms (fatigue, weakness, bone pain, kidney stones, and osteoporosis) are present, even if phosphorus levels are normal or mildly elevated. Monitoring serum calcium, phosphorus, and alkaline phosphatase levels helps to confirm the diagnosis.