**Core Concept**
Parathormone (PTH), also known as parathyroid hormone, plays a crucial role in regulating calcium and phosphate levels in the blood. It acts on the bones, kidneys, and intestines to maintain calcium homeostasis.

**Why the Correct Answer is Right**
PTH increases phosphate excretion in the urine by stimulating the kidneys to secrete phosphate into the urine. This action is mediated through the activation of adenylate cyclase in the renal tubular cells, leading to an increase in cyclic AMP (cAMP) levels. Elevated cAMP levels activate protein kinase A, which in turn phosphorylates and inhibits the sodium-phosphate cotransporter type IIa (NaPi2a) in the proximal convoluted tubules. This inhibition results in decreased phosphate reabsorption and increased phosphate excretion.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the action of PTH on phosphate levels. While PTH does stimulate vitamin D synthesis, which can indirectly affect phosphate levels, this is not the primary mechanism by which PTH regulates phosphate.

**Option B:** This option is incorrect because PTH does not decrease phosphate excretion. Instead, it increases phosphate excretion by inhibiting phosphate reabsorption in the kidneys.

**Option C:** This option is not a direct mechanism by which PTH regulates phosphate levels. While PTH can affect bone resorption, which can release phosphate into the blood, this is not the primary mechanism by which PTH regulates phosphate homeostasis.

**Clinical Pearl / High-Yield Fact**
Remember that PTH regulates phosphate levels by inhibiting phosphate reabsorption in the kidneys, leading to increased phosphate excretion in the urine. This is a critical mechanism for maintaining calcium homeostasis and preventing hyperphosphatemia.

**Correct Answer: B. PTH decreases phosphate reabsorption in the kidneys, leading to increased phosphate excretion.**