**Core Concept**
Metabolic syndrome X, also known as insulin resistance syndrome, is a cluster of conditions that occur together, increasing the risk of developing type 2 diabetes and cardiovascular disease. It is characterized by a combination of central obesity, hypertension, dyslipidemia, and insulin resistance.

**Why the Correct Answer is Right**
Metabolic syndrome X is defined by the World Health Organization (WHO) as a combination of central obesity, defined as a waist circumference of 102 cm or more in men and 88 cm or more in women, hypertension (blood pressure ≥ 130/85 mmHg), elevated fasting glucose (≥ 6.1 mmol/L), and dyslipidemia (elevated triglycerides and low HDL cholesterol). Insulin resistance is a key component of the syndrome, leading to impaired glucose tolerance and increased risk of type 2 diabetes.

**Why Each Wrong Option is Incorrect**
**Option A:** While insulin resistance is a key component of metabolic syndrome X, it is not one of the diagnostic criteria. Insulin resistance is a pathophysiological mechanism underlying the syndrome, but it is not a clinical feature used to diagnose it.
**Option B:** Central obesity is a key component of metabolic syndrome X, characterized by an increased waist circumference. This option is correct, but we are looking for an option that is "EXCEPT".
**Option C:** Hypertension is a component of metabolic syndrome X, characterized by a blood pressure of 130/85 mmHg or higher. This option is correct, but we are looking for an option that is "EXCEPT".
**Option D:** Dyslipidemia, characterized by elevated triglycerides and low HDL cholesterol, is a component of metabolic syndrome X. This option is correct, but we are looking for an option that is "EXCEPT".

**Clinical Pearl / High-Yield Fact**
To remember the components of metabolic syndrome X, use the mnemonic "C-H-E-D": Central Obesity, Hypertension, Elevated Fasting Glucose, and Dyslipidemia.

**Correct Answer: D. Dyslipidemia is not one of the diagnostic criteria for metabolic syndrome X, though it is a common comorbidity in patients with the syndrome.**