**Core Concept**
Bone remodeling is a continuous process in which mature bone tissue is replaced by new bone tissue. This process involves both bone resorption by osteoclasts and bone formation by osteoblasts. The systemic factors that regulate bone remodeling are crucial for maintaining bone health and preventing diseases such as osteoporosis.

**Why the Correct Answer is Right**
Parathyroid hormone (PTH) is a key systemic factor that regulates bone remodeling. It acts on the osteoclasts to increase bone resorption, which in turn stimulates the osteoblasts to form new bone tissue. PTH also regulates calcium levels in the blood by increasing calcium absorption in the intestine and reducing its excretion in the urine. The osteoblasts respond to PTH by releasing osteocalcin, which is a marker of bone formation.

**Why Each Wrong Option is Incorrect**
**Option A:** 1,25-Dihydroxyvitamin D (calcitriol) is a hormone that regulates calcium absorption in the intestine, but it is not the primary systemic factor responsible for bone remodeling. It does, however, work in conjunction with PTH to regulate calcium levels in the blood.
**Option B:** Prostaglandins are local factors that play a role in bone remodeling, but they are not the primary systemic factor responsible for this process. They do, however, interact with systemic factors like PTH to regulate bone metabolism.
**Option C:** Growth hormone (GH) stimulates bone growth and development, but it is not the primary systemic factor responsible for bone remodeling. It does, however, work in conjunction with PTH to regulate bone metabolism.

**Clinical Pearl / High-Yield Fact**
PTH and vitamin D levels should be monitored in patients with osteoporosis to ensure that they are receiving adequate treatment. PTH levels can be elevated in patients with primary hyperparathyroidism, which can lead to bone resorption and osteoporosis.

**Correct Answer: D. Parathyroid hormone (PTH)**