True statement regarding parathyroid glands is
## Core Concept
The parathyroid glands are small endocrine glands located in the neck, playing a crucial role in regulating calcium levels in the blood. They produce **parathyroid hormone (PTH)**, which acts to increase calcium levels by stimulating its release from bones, increasing its absorption in the gut indirectly (through vitamin D activation), and reducing its excretion by the kidneys.
## Why the Correct Answer is Right
The correct answer, **D.**, isn't directly provided, but we can infer based on common facts about parathyroid glands. Typically, a true statement could involve the fact that parathyroid glands are located on the posterior aspect of the thyroid gland, and there are usually four glands. More specifically, PTH acts on **osteoclasts** indirectly to increase bone resorption, increases calcium reabsorption in the **distal convoluted tubule** of the kidneys, and promotes the activation of **vitamin D** to increase intestinal absorption of calcium.
## Why Each Wrong Option is Incorrect
- **Option A:** Without the specific content, we can't directly address why A is incorrect, but common misconceptions could involve the number of parathyroid glands (usually four), their location (typically on the posterior aspect of the thyroid gland), or their function (primarily producing PTH to regulate calcium).
- **Option B:** Similarly, without specifics, B could be incorrect based on inaccurate details about parathyroid hormone's mechanism of action, such as suggesting it directly acts on osteoblasts to decrease bone resorption or inaccurately describing its effect on renal handling of calcium.
- **Option C:** This could be incorrect based on misinformation about the embryological origin of the parathyroid glands (which are derived from the third and second pharyngeal pouches) or their role in disease states like hyperparathyroidism.
## Clinical Pearl / High-Yield Fact
A key point to remember is that **hyperparathyroidism** (excess PTH) can lead to **hypercalcemia** (elevated blood calcium levels), which presents with symptoms like "stones, bones, abdominal moans, and psychic groans." Conversely, **hypoparathyroidism** leads to **hypocalcemia**, causing muscle cramps, tetany, and seizures.
## Correct Answer Line
**Correct Answer: D.**