A 58-year-old man with diabetes mellitus type 2 and hypertension presents to the emergency department with acute kidney injury. His serum creatinine has risen from baseline 1.2 mg/dL to 3.8 mg/dL over 48 hours. Urinalysis shows muddy brown casts and fractional excretion of sodium (FENa) is 2.1%. What is the most common cause of acute kidney injury in hospitalized patients in India?

## Most Common Cause of AKI in Hospitalized Patients **Key Point:** Acute tubular necrosis (ATN) accounts for 45–50% of all cases of acute kidney injury in hospitalized patients globally and in India. ### Pathophysiology of ATN ATN results from damage to the tubular epithelial cells, most commonly due to: 1. **Ischemic injury** (most common, ~50% of ATN cases) — from hypotension, sepsis, major surgery 2. **Nephrotoxic injury** — from aminoglycosides, contrast agents, myoglobin (rhabdomyolysis), hemoglobin (massive hemolysis) ### Clinical and Laboratory Features Supporting ATN in This Case | Feature | Interpretation | |---------|----------------| | Muddy brown casts | Pathognomonic for ATN; composed of sloughed tubular epithelial cells and myoglobin/hemoglobin | | FENa 2.1% | >1% indicates intrinsic renal (tubular) damage; rules out prerenal causes | | Rapid rise in creatinine | Consistent with acute tubular necrosis timeline | | Hospitalized patient | ATN is the leading cause of AKI in hospital settings | **High-Yield:** The combination of **muddy brown casts + FENa >1%** is virtually diagnostic of ATN. ### Epidemiology in India ATN remains the most common cause of AKI in: - Hospitalized patients (45–50%) - Sepsis-related AKI - Post-operative AKI - Rhabdomyolysis-related AKI (common in crush injuries, heat stroke) **Clinical Pearl:** In the Indian context, sepsis-induced ATN is particularly common due to the high burden of infectious diseases and delayed presentation to tertiary care. ### Differential Diagnosis of AKI by FENa | Cause | FENa | Urine Osmolality | Urine Sodium | |-------|------|------------------|---------------| | Prerenal azotemia | <1% | >500 mOsm/kg | <20 mEq/L | | **Acute tubular necrosis** | **>2%** | **<350 mOsm/kg** | **>40 mEq/L** | | Acute interstitial nephritis | Variable (1–3%) | Variable | Variable | | Acute glomerulonephritis | <1% | >500 mOsm/kg | <20 mEq/L | **Mnemonic:** **MUDDY BROWN = ATN** — Muddy brown casts are pathognomonic.