**Core Concept:**
Cushing syndrome is a hormonal disorder that results from sustained exposure to excessive levels of cortisol, a glucocorticoid hormone produced by the adrenal glands. It is characterized by the following features:
1. Adrenal hyperplasia (enlargement and hyperfunctioning of the adrenal cortex)
2. Hypercortisolism (excess cortisol production)
3. Reduced aldosterone production (hypoadrenalism)
4. Hyperglycaemia (high blood glucose levels)

**Why the Correct Answer is Right:**
The correct answer (D) refers to **hypoadrenalism** (reduced aldosterone production) as a feature of Cushing syndrome. This is incorrect because, in Cushing syndrome, the adrenal glands become hyperplastic and hyperfunctioning, leading to **hypercortisolism** (excess cortisol production) rather than hypocortisolism (low cortisol production). The adrenal glands do not produce aldosterone, which is a mineralocorticoid hormone regulated by the Hypothalamic-Pituitary-Adrenal (HPA) axis. Instead, it is the HPA axis that is disrupted in Cushing syndrome, causing the symptoms and signs associated with hypercortisolism, not hypocortisolism.

**Why Each Wrong Option is Incorrect:**

A. **Hypokalaemia** (low potassium levels) is a feature of Cushing syndrome. An elevated cortisol level can lead to a loss of potassium from the body, contributing to hypokalaemia.

B. **Peripheral resistance to cortisol** is not a feature of Cushing syndrome. While cortisol resistance might occur in secondary adrenal insufficiency, Cushing syndrome is characterized by hypercortisolism, not resistance to cortisol.

C. **Hypothyroidism** (low thyroid function) is not a feature of Cushing syndrome. Hypothyroidism is a condition caused by underactive thyroid, whereas Cushing syndrome is a result of excess cortisol production.

**Clinical Pearl:**

In Cushing syndrome, it is essential to differentiate between the signs and symptoms of hypercortisolism (excess cortisol) and hypocortisolism (low cortisol). Hypokalaemia and hypercortisolism are common presentations, while hypothyroidism and peripheral resistance to cortisol are not specific to Cushing syndrome. A thorough understanding of the pathophysiology, clinical features, and differential diagnosis is crucial to accurately diagnose and manage this condition.