**Core Concept**
Topical steroids are widely used for their anti-inflammatory and immunosuppressive properties in the treatment of various skin conditions. However, their chronic and excessive use can lead to several complications due to the suppression of the hypothalamic-pituitary-adrenal (HPA) axis and local tissue effects.

**Why the Correct Answer is Right**
The commonest complication of topical steroids is the development of **perioral dermatitis** and **trophic changes**, including skin atrophy, telangiectasia, and striae. This occurs due to the prolonged suppression of the HPA axis, leading to a decrease in cortisol production, which can result in the thinning of the skin, loss of fat, and reduced collagen synthesis. Additionally, the local effects of topical steroids can cause vasoconstriction and reduce the production of collagen and elastin, leading to skin thinning and atrophy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rebound hyperglycemia is not a common complication of topical steroids. While systemic steroids can cause hyperglycemia, topical steroids have a negligible effect on glucose metabolism.
* **Option B:** Contact dermatitis is an adverse reaction to topical steroids, but it is not the most common complication. Topical steroids can cause contact dermatitis in sensitive individuals, but this is not as common as the other options listed.
* **Option C:** Osteoporosis is a complication of systemic steroids, not topical steroids. Systemic steroids can cause bone resorption and decrease bone density, leading to osteoporosis, but topical steroids have a negligible effect on bone metabolism.

**Clinical Pearl / High-Yield Fact**
Topical steroids should be used with caution and for the shortest duration necessary to avoid the complications associated with their chronic use. It's essential to use the lowest potency steroid possible and apply it to the smallest area necessary to treat the condition.

**Correct Answer: C. Perioral dermatitis and trophic changes.