**Core Concept**
Increased plasma concentration of calcium is associated with conditions that either lead to excessive calcium absorption, decreased calcium excretion, or increased bone resorption. Primary hyperparathyroidism is a disorder characterized by excessive secretion of parathyroid hormone (PTH), which plays a crucial role in regulating calcium homeostasis.

**Why the Correct Answer is Right**
In primary hyperparathyroidism, the parathyroid glands secrete excess PTH, leading to increased calcium levels in the blood. PTH acts on the bones to stimulate bone resorption, releasing calcium into the bloodstream. Additionally, PTH promotes calcium reabsorption in the kidneys and increases calcium absorption from the gut. This complex interplay results in elevated plasma calcium concentrations.

**Why Each Wrong Option is Incorrect**
**Option A:** Rickets is a condition characterized by softening of the bones in children due to vitamin D deficiency or phosphate deficiency, leading to decreased calcium levels in the blood.
**Option B:** Osteomalacia is a condition similar to rickets but occurs in adults, also resulting from vitamin D deficiency or phosphate deficiency, leading to decreased calcium levels in the blood.
**Option D:** Pseudohypoparathyroidism is a rare genetic disorder characterized by resistance to PTH, leading to decreased calcium levels in the blood.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of primary hyperparathyroidism is the presence of kidney stones, which are often composed of calcium oxalate due to increased calcium levels in the urine.

**✓ Correct Answer: C. Primary hyperparathyroidism**