**Core Concept:** Chronic kidney disease (CKD) is a spectrum of kidney damage lasting for more than three months, leading to a decline in kidney function. Glomerular filtration rate (GFR) is a crucial parameter to assess kidney function. The CKD classification is based on GFR, which is determined by dividing serum creatinine by the formula: Cockroft-Gault, MDRD (Modification of Diet in Renal Disease) or CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration).

**Why the Correct Answer is Right:** Chronic kidney disease is defined as a glomerular filtration rate (GFR) less than 60 mL/min/1.73 m^2 for three months or more. This classification system is based on the GFR formula called Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI), which is widely used in clinical practice. The CKD-EPI formula takes into account age, sex, race, and creatinine levels to provide a more accurate estimation of GFR compared to the Cockcroft-Gault and MDRD formulas.

**Why Each Wrong Option is Incorrect:**

A. **GFR less than 15 mL/min/1.73 m^2**: This is too low, indicating an end-stage renal disease (ESRD), which is a more advanced stage than CKD. The GFR should be less than 60 mL/min/1.73 m^2 for CKD classification.

B. **GFR less than 30 mL/min/1.73 m^2**: This is also too low, indicating a more severe stage of CKD than the correct answer. The GFR should be less than 60 mL/min/1.73 m^2 for CKD classification.

C. **GFR less than 45 mL/min/1.73 m^2**: This value is slightly higher than the correct answer but still falls within a more severe stage of CKD. The GFR should be less than 60 mL/min/1.73 m^2 for CKD classification.

D. **GFR less than 90 mL/min/1.73 m^2**: This value is higher than the correct answer and indicates normal kidney function. The GFR should be less than 60 mL/min/1.73 m^2 for CKD classification.

**Clinical Pearl:** The CKD classification is essential to guide patient management, prognosis, and decision-making regarding renal replacement therapy. It is crucial for early detection and management of complications, and monitoring of the disease progression. In addition, the classification helps in identifying patients at risk for cardiovascular events and death.

**Correct Answer:** GFR less than 60 mL/min/1.73 m^2 indicates early-stage CKD, and the classification helps in monitoring the disease progression and identifying patients at risk for cardiovascular events and death.