Contrast-induced nephropathy (CIN), now termed contrast-associated acute kidney injury (CA-AKI), is acute renal injury occurring within 48-72 hours of intravascular iodinated contrast administration. The pathophysiology involves two mechanisms: direct tubular cytotoxicity from the hyperosmolar contrast agent causing oxidative stress, mitochondrial dysfunction, and apoptosis of renal tubular epithelial cells; and renal medullary ischemia from contrast-induced vasoconstriction (mediated by endothelin, adenosine, and reduced nitric oxide) in the already hypoxic outer medulla. CIN is defined as a serum creatinine increase of ≥0.3 mg/dL or ≥50% from baseline within 48-72 hours of contrast exposure.