PTH does not cause –
**Core Concept**
Parathyroid hormone (PTH) is a crucial regulator of calcium and phosphorus homeostasis in the body. It acts by stimulating the release of calcium from bones, increasing calcium absorption in the gut, and promoting the excretion of phosphorus in the urine.
**Why the Correct Answer is Right**
PTH has a biphasic effect on bone resorption, with an initial stimulatory effect followed by an inhibitory effect. This biphasic effect is due to the stimulation of osteoclast activity, which leads to increased bone resorption, and the subsequent inhibition of osteoclast activity, leading to a decrease in bone resorption. This dual effect is mediated by the activation of PTH receptors on osteoblasts, which in turn regulate osteoclast activity.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because PTH does indeed cause an increase in bone resorption, which leads to an increase in calcium levels in the blood.
**Option B:** This option is incorrect because PTH actually increases calcium absorption in the gut, not decreases it.
**Option C:** This option is incorrect because PTH promotes the excretion of phosphorus in the urine, not the retention of phosphorus.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that PTH has a biphasic effect on bone resorption, which can lead to an increase in bone density in the short term but ultimately result in bone loss over time.
**Correct Answer:** D.