A 32-year-old man is brought to the emergency department after a car accident; he was extricated after 4 hours. He did not lose consciousness and does not have headache or nausea. He is in severe pain. He sustained severe injuries to both arms and the trauma team determines that surgical intervention is needed. Urinary catheterization shows dark colored urine. His temperature is 38°C (100.4°F), pulse is 110/min, and blood pressure is 90/60 mm Hg. The patient is alert and oriented. Examination shows multiple injuries to the upper extremities, contusions on the trunk, and abdominal tenderness. Laboratory studies show: Hemoglobin 9.2 g/dL Leukocyte count 10,900/mm3 Platelet count 310,000/mm3 Serum Na+ 137 mEq/L K+ 6.8 mEq/L Cl- 97 mEq/L Glucose 168 mg/dL Creatinine 1.7 mg/dL Calcium 7.7 mg/dL Arterial blood gas analysis on room air shows a pH of 7.30 and a serum bicarbonate of 14 mEq/L. An ECG shows peaked T waves. A FAST scan of the abdomen is negative. Two large bore cannulas are inserted and intravenous fluids are administered. Which of the following is the most appropriate next step in management?

Packed red blood cell transfusion

A 52-year-old man is brought to the emergency department by ambulance after a motor vehicle accident. He was an unrestrained passenger who was ejected from the vehicle. On presentation, he is found to be actively bleeding from numerous wounds. His blood pressure is 76/42 mmHg and pulse is 152/min. Attempts at resuscitation fail, and he dies 25 minutes later. Autopsy shows blood in the peritoneal cavity, and histology of the kidney reveals swelling of the proximal convoluted tubule epithelial cells. Which of the following is most likely the mechanism underlying the renal cell findings?

Decreased function of the Na+/K+-ATPase

Decreased activity of caspase 7

Increased activity of caspase 9

Increased function of the Na+/K+-ATPase

Increased activity of caspase 8

A 67-year-old man comes to the emergency department for the evaluation of two episodes of red urine since this morning. He has no pain with urination. He reports lower back pain and fever. Six months ago, he was diagnosed with osteoarthritis of the right knee that he manages with 1–2 tablets of ibuprofen per day. He has smoked one pack of cigarettes daily for the past 45 years. He does not drink alcohol. His temperature is 38.5°C (101.3°F), pulse is 95/min, and blood pressure is 130/80 mm Hg. Physical examination shows faint, diffuse maculopapular rash, and bilateral flank pain. The remainder of the examination shows no abnormalities. Urinalysis shows: Blood +3 Protein +1 RBC 10–12/hpf RBC cast negative Eosinophils numerous Which of the following is the most likely diagnosis?

Acute tubulointerstitial nephritis

Crystal-induced acute kidney injury

A 48-year-old man is brought to the emergency department by his wife because of a 3-day history of increasing confusion and lethargy. He complains of decreased urine output and abdominal pain for the past month. Two months ago, he was hospitalized for pyelonephritis and treated with ceftriaxone. He has a history of chronic hepatitis C. He does not take any medications. He appears pale and irritable. His temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 98/60 mm Hg. On mental status examination, he is oriented to person but not to time or place. Physical examination shows scleral icterus and jaundice. There is 2+ pitting edema of the lower extremities. The abdomen is distended with a positive fluid wave. Laboratory studies show: Hemoglobin 10.1 g/dL Leukocyte count 4300/mm3 Platelet count 89,000/mm3 Prothrombin time 19 sec Serum Urea nitrogen 71 mg/dL Glucose 99 mg/dL Creatinine 3.5 mg/dL ALT 137 mg/dL AST 154 mg/dL Urinalysis shows no abnormalities. The FeNa is < 1%. Ultrasound of the kidneys is unremarkable. Intravenous fluids are administered for 36 hours but do not improve urine output. Which of the following is the most likely cause of the kidney dysfunction in this patient?

Renal interstitial inflammation

A 32-year-old man is brought to the Emergency Department after 3 consecutive days of diarrhea, fatigue and weakness. His stool has been soft and mucoid, with no blood stains. The patient just came back from a volunteer mission in Guatemala, where he remained asymptomatic. His personal medical history is unremarkable. Today his blood pressure is 98/60 mm Hg, pulse is 110/min, respiratory rate is 19/min, and his body temperature is 36.7°C (98.1°F). On physical exam, he has sunken eyes, dry mucosa, mild diffuse abdominal tenderness, and hyperactive bowel sounds. Initial laboratory tests are shown below: Serum creatinine (SCr) 1.8 mg/dL Blood urea nitrogen (BUN) 50 mg/dL Serum sodium 132 mEq/L Serum potassium 3.5 mEq/L Serum chloride 102 mEq/L Which of the following phenomena would you expect in this patient?

High urine osmolality, low FeNa+, low urine Na+

Low urine osmolality, high FeNa+, high urine Na+

High urine osmolality, high fractional excretion of sodium (FeNa+), high urine Na+

Low urine osmolality, high FeNa+, low urine Na+

Low urine osmolality, low FeNa+, high urine Na+

A 63-year-old man undergoes uncomplicated laparoscopic cholecystectomy for acute cholecystitis and is admitted to the surgical ward for postoperative management. On postoperative day 1, routine laboratory studies reveal an increase in serum creatinine to 1.46 mg/dL from 0.98 mg/dL before the operation; BUN is 37 mg/dL, increased from 18 mg/dL on prior measurement; K is 4.8 mEq/L and CO2 is 19 mEq/L. The patient has an indwelling urinary catheter in place, draining minimal urine over the last few hours. Which of the following is the most appropriate next step in management?

Evaluate urinary catheter for obstruction

Obtain bilateral renal ultrasonography

A 45-year-old homeless man is brought to the emergency department by the police. He was found intoxicated and passed out in a library. The patient has a past medical history of IV drug abuse, diabetes, alcohol abuse, and malnutrition. The patient has been hospitalized previously for multiple episodes of pancreatitis and sepsis. Currently, the patient is minimally responsive and only withdraws his extremities in response to painful stimuli. His temperature is 99.5°F (37.5°C), blood pressure is 90/48 mmHg, pulse is 150/min, respirations are 17/min, and oxygen saturation is 95% on room air. Physical exam is notable for tachycardia, a diastolic murmur at the left lower sternal border, and bilateral crackles on pulmonary exam. The patient is started on IV fluids, vancomycin, and piperacillin-tazobactam. Laboratory values are ordered as seen below. Hemoglobin: 9 g/dL Hematocrit: 30% Leukocyte count: 11,500/mm^3 with normal differential Platelet count: 297,000/mm^3 Serum: Na+: 139 mEq/L Cl-: 100 mEq/L K+: 4.0 mEq/L HCO3-: 28 mEq/L BUN: 33 mg/dL Glucose: 60 mg/dL Creatinine: 1.7 mg/dL Ca2+: 9.7 mg/dL PT: 20 seconds aPTT: 60 seconds AST: 1,010 U/L ALT: 950 U/L The patient is admitted to the medical floor. Five days later, the patient's neurological status has improved. His temperature is 99.5°F (37.5°C), blood pressure is 130/90 mmHg, pulse is 90/min, respirations are 11/min, and oxygen saturation is 99% on room air. Laboratory values are repeated as seen below. Hemoglobin: 10 g/dL Hematocrit: 32% Leukocyte count: 9,500/mm^3 with normal differential Platelet count: 199,000/mm^3 Serum: Na+: 140 mEq/L Cl-: 102 mEq/L K+: 4.3 mEq/L HCO3-: 24 mEq/L BUN: 31 mg/dL Glucose: 100 mg/dL Creatinine: 1.6 mg/dL Ca2+: 9.0 mg/dL PT: 40 seconds aPTT: 90 seconds AST: 150 U/L ALT: 90 U/L Which of the following is the best description of this patient’s current status?

Recovery from acute renal failure

Recovery from ischemic liver disease

Recovery from acute alcoholic liver disease

Acute kidney injury for USMLE Step 3: High-Yield Internal Medicine Lessons

AKI Fundamentals - The Kidney Crisis

Definition: Abrupt kidney dysfunction causing ↑ creatinine & ↓ urine output.
- ↑ SCr by ≥0.3 mg/dL within 48h
- ↑ SCr to ≥1.5x baseline
- Urine output <0.5 mL/kg/h for >6 hours
Types:
- Prerenal: Hypoperfusion (e.g., dehydration). BUN/Cr >20, $FeNa$ <1%.
- Intrinsic: Direct kidney damage (e.g., ATN, AIN).
- Postrenal: Urinary tract obstruction (e.g., BPH, stones).

Acute Kidney Injury: Causes, Classification, & Complications

⭐ Fractional excretion of sodium ($FeNa$) is a key test to differentiate prerenal AKI from ATN, but its utility is limited in patients on diuretics.

Prerenal AKI - The Dry Spell

Pathophysiology: ↓ Renal perfusion triggers physiologic Na⁺/H₂O retention. It is typically reversible with restored blood flow.
Etiologies:
- True volume loss (e.g., hemorrhage, dehydration).
- ↓ Effective circulating volume (e.g., heart failure, cirrhosis).
- Renal artery stenosis.
- Drugs: NSAIDs, ACE inhibitors, ARBs.
Key Labs:
- BUN/Cr Ratio > 20:1
- Urine Osmolality > 500 mOsm/kg
- FENa < 1%
- Urine Na⁺ < 20 mEq/L

⭐ The hallmark is a BUN/Cr ratio > 20:1, reflecting increased passive urea reabsorption in the proximal tubule due to enhanced sodium and water reabsorption.

Intrinsic AKI - Muddy Waters

Direct damage to kidney structures (tubules, interstitium, glomeruli). Characterized by FENa > 2% and Urine Osm < 350 mOsm/kg.

Acute Tubular Necrosis (ATN)

Most common cause.
- Ischemic: Prolonged hypotension/hypoxia (e.g., shock, sepsis).
- Nephrotoxic: Aminoglycosides, contrast dye, myoglobin, ethylene glycol.
Urinalysis: Muddy brown granular casts are pathognomonic.

Acute Interstitial Nephritis (AIN)

Inflammatory infiltrate in the interstitium, often drug-induced.
- Classic Triad: Fever, rash, arthralgias.
- Urinalysis: WBC casts, sterile pyuria, and eosinophiluria.

⭐ Muddy brown casts in ATN are composed of necrotic tubular epithelial cells and Tamm-Horsfall protein.

Urinary casts and associated kidney conditions

Postrenal AKI - The Block Party

Pathophysiology: Urinary tract obstruction → ↑ intratubular pressure → ↓ GFR.
Etiologies: Benign Prostatic Hyperplasia (BPH) is the most common cause. Others include nephrolithiasis, tumors (prostate, cervical), and neurogenic bladder.
Diagnosis:
- Bladder ultrasound is the initial test to look for hydronephrosis.
- Catheterization can be both diagnostic and therapeutic.
Labs: Early phase shows BUN:Cr > 15:1.

⭐ Unilateral obstruction does not cause AKI unless the patient has only one functioning kidney.

Acute Kidney Injury (AKI) Quick Facts Infographic

AKI Management - Dialysis Decisions

Indications for urgent dialysis (📌 AEIOU):

Acidosis: Severe, refractory metabolic acidosis (pH < 7.1)
Electrolytes: Severe, refractory hyperkalemia (K⁺ > 6.5 mEq/L)
Intoxication: Specific poisons (e.g., salicylates, lithium, methanol, ethylene glycol)
Overload: Refractory fluid overload causing pulmonary edema
Uremia: Symptomatic (encephalopathy, seizures, or pericarditis)

⭐ Uremic pericarditis is an absolute indication for immediate dialysis.

High‑Yield Points - ⚡ Biggest Takeaways

Prerenal AKI, the most common type, shows a BUN/Cr ratio > 20:1 and FeNa < 1%, often from hypovolemia.

Acute Tubular Necrosis (ATN), the top intrinsic cause, presents with muddy brown granular casts and an FeNa > 2%.

Postrenal AKI results from obstruction; diagnose with a renal ultrasound looking for hydronephrosis.

Cardiorenal and hepatorenal syndromes are critical diagnoses in advanced heart failure and cirrhosis, respectively.

Always review medications for nephrotoxins like NSAIDs and contrast dye, especially in high-risk settings.

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