**Core Concept:**
The question is asking about the direct effects of Parathyroid Hormone (PTH) on the body. PTH is a hormone secreted by the parathyroid glands primarily aiming to maintain calcium and phosphorus levels in the blood within a normal range.

**Why the Correct Answer is Right:**
PTH directly acts on three primary target tissues: bones, kidneys, and the gastrointestinal tract. It increases bone resorption (release of calcium from bone matrix) and renal calcium reabsorption, ultimately leading to elevated serum calcium levels. Additionally, PTH stimulates vitamin D activation (1α-hydroxylation) in the kidney, which further enhances calcium absorption from the gut, thus increasing serum calcium levels.

**Why Each Wrong Option is Incorrect:**

A. **Incorrect answer**: PTH does not directly affect the liver. Its primary effects are on bones, kidneys, and the gut, as explained above.

B. **Incorrect answer**: Although PTH indirectly affects renal phosphate excretion by enhancing 1α-hydroxylation, it does not directly decrease phosphate reabsorption.

C. **Incorrect answer**: PTH does not directly influence thyroid function; it primarily works on bones, kidneys, and the gut.

D. **Incorrect answer**: PTH does not directly affect erythropoiesis; it primarily influences calcium homeostasis through actions on bones, kidneys, and the gut.

**Clinical Pearl / High-Yield Fact:**
A good way to remember the primary actions of PTH is the mnemonic "BONE": Bones, Ovaries, Nephrons, and Erythropoiesis. While PTH indirectly influences some aspects of these organ systems, it does not directly affect them as stated above.

**Correct Answer:** D. PTH directly affects bones, kidneys, and the gastrointestinal tract. It increases bone resorption, enhances renal calcium reabsorption, and stimulates vitamin D activation in the kidney to promote calcium absorption in the gut.