A 56-year-old man is brought to the Emergency Department with intense chest pain that radiates to his left arm and jaw. He also complains of feeling lightheaded. Upon arrival, his blood pressure is 104/60 mm Hg, pulse is 102/min, respiratory rate is 25/min, body temperature is 36.5°C (97.7°F), and oxygen saturation is 94% on room air. An electrocardiogram shows an ST-segment elevation in I, aVL, and V5-6. The patient is transferred to the cardiac interventional suite for a percutaneous coronary intervention. The patient is admitted to the hospital after successful revascularization. During his first night on the ICU floor his urinary output is 0.15 mL/kg/h. Urinalysis shows muddy brown casts. Which of the following outcomes specific to the patient’s condition would you expect to find?

Blood urea nitrogen (BUN):Serum creatinine ratio (Cr) < 15:1

Urinary osmolality 900 mOsmol/kg (normal: 500–800 mOsmol/kg)

Urinary osmolality 550 mOsmol/kg (normal: 500–800 mOsmol/kg)

Blood urea nitrogen (BUN):Serum creatinine ratio (Cr) > 20:1

A 33-year-old man presents to the emergency department because of an episode of bloody emesis. He has had increasing dyspnea over the past 2 days. He was diagnosed with peptic ulcer disease last year. He has been on regular hemodialysis for the past 2 years because of end-stage renal disease. He skipped his last dialysis session because of an unexpected business trip. He has no history of liver disease. His supine blood pressure is 110/80 mm Hg and upright is 90/70, pulse is 110/min, respirations are 22/min, and temperature is 36.2°C (97.2°F). The distal extremities are cold to touch, and the outstretched hand shows flapping tremor. A bloody nasogastric lavage is also noted, which eventually clears after saline irrigation. Intravenous isotonic saline and high-dose proton pump inhibitors are initiated, and the patient is admitted into the intensive care unit. Which of the following is the most appropriate next step in the management of this patient?

Observation in the intensive care unit

Transfusion of packed red blood cells

An 84-year-old man is brought to the physician by the staff of a group home where he resides because of worsening confusion and decreased urinary output. His nurse reports that the patient has not been drinking much for the last 3 days. Examination shows a decreased skin turgor and dry oral mucosa. His pulse is 105/min and blood pressure is 100/65 mm Hg. His serum creatinine is 3.1 mg/dL and a urea nitrogen is 42 mg/dL. Urine studies show multiple brownish granular casts. Which of the following processes is most likely involved in the pathogenesis of this patient's condition?

Necrosis of tubular epithelial cells

Immune complex deposition in mesangium

Leukocytic infiltration of renal interstitium

Disruption of glomerular podocytes

An asymptomatic 15-year-old high school wrestler with no family history of renal disease is completing his preseason physical exam. He submits a urine sample for a dipstick examination, which tests positive for protein. What is the next appropriate step in management?

Repeat dipstick on a separate occasion

Spot urine-protein-to-creatinine ratio

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 25-year-old man is brought to the emergency department because of a 6-day history of fever and chills. During this period, he has had generalized weakness, chest pain, and night sweats. He has a bicuspid aortic valve and recurrent migraine attacks. He has smoked one pack of cigarettes daily for 5 years. He does not drink alcohol. He has experimented with intravenous drugs in the past but has not used any illicit drugs in the last two months. Current medications include propranolol and a multivitamin. He appears ill. His temperature is 39°C (102.2°F), pulse is 108/min, respirations are 14/min, and blood pressure is 150/50 mm Hg. Diffuse crackles are heard. A grade 3/6 high-pitched, early diastolic, decrescendo murmur is best heard along the left sternal border. An S3 gallop is heard. The remainder of the physical examination shows no abnormalities. Laboratory studies show: Hemoglobin 13.1 g/dL Leukocyte count 13,300/mm3 Platelet count 270,000/mm3 Serum Glucose 92 mg/dL Creatinine 0.9 mg/dL Total bilirubin 0.4 mg/dL AST 25 U/L ALT 28 U/L Three sets of blood cultures are sent to the laboratory. Transthoracic echocardiography confirms the diagnosis. In addition to antibiotic therapy, which of the following is the most appropriate next step in management?

Porcine valve replacement of the aortic valve

Transcatheter aortic valve implantation (TAVI)

Repeat echocardiography in 4 weeks

Mechanical valve replacement of the aortic valve

Acute renal failure management for USMLE Step 2 CK: High-Yield Management Lessons

ARF: Definition & Diagnosis - Kidney Crisis Kickstart

Definition: Abrupt decline in kidney function, leading to the retention of urea and other nitrogenous waste products.
KDIGO Diagnostic Criteria (1 of 3):
- ↑ SCr by ≥0.3 mg/dL within 48 hours
- ↑ SCr to ≥1.5x baseline within 7 days
- Urine volume <0.5 mL/kg/h for 6 hours
Initial Workup: BMP, urinalysis with microscopy, and renal ultrasound to rule out post-renal obstruction.

⭐ Fractional excretion of sodium ($FeNa$) is a key test to differentiate pre-renal azotemia ($FeNa$ <1%) from acute tubular necrosis (ATN) ($FeNa$ >2%).

ARF: Etiology - The Usual Suspects

Pre-renal (Hypoperfusion)
- Causes: Hypovolemia (hemorrhage, dehydration), ↓ cardiac output (heart failure), sepsis, renal artery stenosis.
- Labs: BUN/Cr > 20:1, FeNa < 1%, concentrated urine (Uosm > 500).
Intrinsic (Kidney Damage)
- Acute Tubular Necrosis (ATN): Most common. Ischemic (prolonged pre-renal) or nephrotoxic (contrast, aminoglycosides, myoglobin). muddy brown casts.
- Acute Interstitial Nephritis (AIN): Allergic reaction. 📌 Pee (diuretics), Pain-free (NSAIDs), Penicillins, PPIs, rifamPin. WBC casts, eosinophils.
- Glomerulonephritis: Hematuria, RBC casts.
Post-renal (Obstruction)
- Causes: BPH, kidney stones, tumors, neurogenic bladder.

⭐ Fractional Excretion of Sodium (FeNa) is key to differentiate pre-renal from ATN. $FeNa = \frac{(P_{Cr} \times U_{Na})}{(P_{Na} \times U_{Cr})} \times 100$. An FeNa < 1% suggests a pre-renal cause, while > 2% points towards ATN.

Causes of Acute Kidney Injury

ARF: Workup & Management - Fixing the Filters

Initial Steps: Discontinue nephrotoxic agents (e.g., NSAIDs, ACE-I). Assess and optimize volume status. Rule out and relieve any urinary obstruction.
Monitoring: Track vital signs, strict I/O, daily weights, and serial chemistry panels (BUN, Cr, K+).

⭐ In oliguric patients, a Fractional Excretion of Sodium ($F_eNa$) <1% strongly suggests a pre-renal state, whereas a $F_eNa$ >2% points towards Acute Tubular Necrosis (ATN). Note: recent diuretic use can render $F_eNa$ unreliable.

ARF: Dialysis Indications - The AEIOU Exits

📌 AEIOU mnemonic for emergent dialysis:

Acidosis: Severe metabolic acidosis (pH < 7.1)
Electrolytes: Refractory hyperkalemia (K⁺ > 6.5 mEq/L) or symptomatic changes
Ingestions: Toxic alcohols (methanol, ethylene glycol), salicylates, lithium
Overload: Refractory volume overload (e.g., pulmonary edema)
Uremia: Symptomatic (encephalopathy, pericarditis, bleeding)

⭐ Uremic pericarditis is an absolute indication for dialysis, regardless of BUN/creatinine levels. It may not show classic diffuse ST elevation on ECG.

AEIOU: Dialysis Indications for Acute Kidney Injury

High-Yield Points - ⚡ Biggest Takeaways

Prerenal azotemia, the most common cause of AKI, typically responds to IV fluids.

Differentiate prerenal from ATN using a BUN/Cr ratio >20 and FeNa <1%.

For postrenal failure, the priority is to relieve the obstruction, often with a Foley catheter.

Immediately discontinue all nephrotoxic agents (e.g., NSAIDs, ACE inhibitors, contrast).

Treat severe hyperkalemia emergently with calcium gluconate, followed by insulin/glucose.

Indications for urgent dialysis follow the AEIOU mnemonic: Acidosis, Electrolytes, Ingestions, Overload, Uremia.

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