**Core Concept**
The question is testing the understanding of the physiological effects of parathyroid hormone (PTH) on calcium metabolism and the consequences of its removal or reduction in the immediate post-operative period.

**Why the Correct Answer is Right**
In the immediate post-operative period following excision of a parathyroid adenoma, hypocalcemia can occur due to the sudden decrease in parathyroid hormone (PTH) levels. PTH plays a crucial role in regulating calcium metabolism by increasing calcium release from bones, increasing calcium absorption from the gut, and decreasing calcium excretion by the kidneys. When a parathyroid adenoma is removed, the excess PTH production is stopped, leading to a decrease in circulating PTH levels. This decrease in PTH levels results in a decrease in bone resorption, reduced calcium absorption from the gut, and increased calcium excretion by the kidneys, ultimately leading to hypocalcemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely a distractor and not related to the physiological effects of PTH removal. Without more information, it's difficult to determine the specific reason why this option is incorrect.

**Option B:** This option might be related to a different condition or a different aspect of calcium metabolism. However, it's not directly related to the removal of a parathyroid adenoma and the subsequent decrease in PTH levels.

**Option C:** This option is incorrect because it's not directly related to the removal of a parathyroid adenoma and the subsequent decrease in PTH levels. While vitamin D is important for calcium absorption, the immediate post-operative period following parathyroid adenoma excision is not typically associated with vitamin D deficiency.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hypocalcemia can occur in the immediate post-operative period following parathyroid adenoma excision due to the sudden decrease in PTH levels. This is a classic exam trap, and students should be aware of the potential for hypocalcemia in this scenario.

**Correct Answer:** D.