## Core Concept
The question tests knowledge of the side effects associated with corticosteroid use. Corticosteroids are widely used for their anti-inflammatory and immunosuppressive properties but come with a range of potential side effects due to their impact on various bodily systems.

## Why the Correct Answer is Right
Osteoporosis is a well-documented side effect of long-term corticosteroid use. Steroids can interfere with the body's calcium balance and bone formation, leading to decreased bone density. This effect is a significant concern in patients on long-term steroid therapy.

## Why Each Wrong Option is Incorrect
* **Option A:** Weight gain is a common side effect of steroids due to increased appetite and redistribution of fat, particularly in the trunk area.
* **Option B:** Hypertension is another side effect, which can be due to fluid retention, increased sodium levels, or the direct effect of steroids on blood vessels.
* **Option D:** Hyperglycemia or diabetes mellitus can occur because steroids induce insulin resistance, making it harder for glucose to enter cells.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the risk of osteoporosis can be mitigated with adequate calcium and vitamin D supplementation, as well as the use of bisphosphonates in high-risk patients. Regular monitoring of bone density is recommended for patients on long-term steroid therapy.

## Correct Answer Line
**Correct Answer: D. Hyperkalemia**