A 52-year-old woman is brought to the emergency department by her husband because of weakness, abdominal pain, and a productive cough for 4 days. She also reports increased urination for the past 2 days. This morning, she had nausea and five episodes of vomiting. She has type 1 diabetes mellitus and hypertension. Current medications include insulin and lisinopril. She admits to have forgotten to take her medication in the last few days. Her temperature is 38.4°C (101.1°F), pulse is 134/min, respirations 31/min, and blood pressure is 95/61 mm Hg. Examination shows dry mucous membranes and decreased skin turgor. Abdominal examination shows diffuse tenderness with no guarding or rebound. Bowel sounds are normal. Laboratory studies show: Serum Na+ 139 mEq/L K+ 5.3 mEq/L Cl- 106 mEq/L Glucose 420 mg/dL Creatinine 1.0 mg/dL Urine Blood negative Glucose 4+ Ketones 3+ Arterial blood gas analysis on room air shows: pH 7.12 pCO2 17 mm Hg pO2 86 mm Hg HCO3- 12 mEq/L Which of the following is the most likely underlying cause of this patient's increased potassium?

Increased renal potassium absorption

A 52-year-old man with a history of Type 1 diabetes mellitus presents to the emergency room with increasing fatigue. Two days ago, he ran out of insulin and has not had time to obtain a new prescription. He denies fevers or chills. His temperature is 37.2 degrees Celsius, blood pressure 84/56 mmHg, heart rate 100/min, respiratory rate 20/min, and SpO2 97% on room air. His physical exam is otherwise within normal limits. An arterial blood gas analysis shows the following: pH 7.25, PCO2 29, PO2 95, HCO3- 15. Which of the following acid-base disorders is present?

Metabolic acidosis with appropriate respiratory compensation

Respiratory alkalosis with appropriate metabolic compensation

Respiratory acidosis with appropriate metabolic compensation

Mixed metabolic and respiratory acidosis

Metabolic alkalosis with appropriate respiratory compensation

A 54-year-old man presents with 3 days of non-bloody and non-bilious emesis every time he eats or drinks. He has become progressively weaker and the emesis has not improved. He denies diarrhea, fever, or chills and thinks his symptoms may be related to a recent event that involved sampling many different foods. His temperature is 97.5°F (36.4°C), blood pressure is 133/82 mmHg, pulse is 105/min, respirations are 15/min, and oxygen saturation is 98% on room air. Physical exam is notable for a weak appearing man with dry mucous membranes. His abdomen is nontender. Which of the following laboratory changes would most likely be seen in this patient?

Metabolic alkalosis and hypokalemia

Metabolic alkalosis and hyperkalemia

Non-anion gap metabolic acidosis and hypokalemia

Respiratory acidosis and hyperkalemia

Anion gap metabolic acidosis and hypokalemia

A 57-year-old woman comes to the emergency department because of dizziness, nausea, and vomiting for 4 days. Her temperature is 37.3°C (99.1°F), pulse is 100/min, respirations are 20/min, and blood pressure is 110/70 mm Hg. Physical examination shows no abnormalities. Arterial blood gas analysis on room air shows: pH 7.58 PCO2 43 mm Hg PO2 96 mm Hg HCO3- 32 mEq/L The most appropriate next step in diagnosis is measurement of which of the following?

Urine albumin to creatinine ratio

A 58-year-old man presents to the emergency department with a chief complaint of ringing in his ears that started several hours previously that has progressed to confusion. The patient denies any history of medical problems except for bilateral knee arthritis. He was recently seen by an orthopedic surgeon to evaluate his bilateral knee arthritis but has opted to not undergo knee replacement and prefers medical management. His wife noted that prior to them going on a hike today, he seemed confused and not himself. They decided to stay home, and roughly 14 hours later, he was no longer making any sense. Physical exam is notable for a confused man. The patient's vitals are being performed and his labs are being drawn. Which of the following is most likely to be seen on blood gas analysis?

pH: 7.30, PaCO2: 15 mmHg, HCO3-: 16 mEq/L

pH: 7.37, PaCO2: 41 mmHg, HCO3-: 12 mEq/L

pH: 7.41, PaCO2: 65 mmHg, HCO3-: 34 mEq/L

pH: 7.47, PaCO2: 11 mmHg, HCO3-: 24 mEq/L

pH: 7.31, PaCO2: 31 mmHg, HCO3-: 15 mEq/L

A 30-year-old man is brought to the emergency room by ambulance after being found unconscious in his car parked in his garage with the engine running. His wife arrives and reveals that his past medical history is significant for severe depression treated with fluoxetine. He is now disoriented to person, place, and time. His temperature is 37.8 deg C (100.0 deg F), blood pressure is 100/50 mmHg, heart rate is 100/min, respiratory rate is 10/min, and SaO2 is 100%. On physical exam, there is no evidence of burn wounds. He has moist mucous membranes and no abnormalities on cardiac and pulmonary auscultation. His respirations are slow but spontaneous. His capillary refill time is 4 seconds. He is started on 100% supplemental oxygen by non-rebreather mask. His preliminary laboratory results are as follows: Arterial blood pH 7.20, PaO2 102 mm Hg, PaCO2 23 mm Hg, HCO3 10 mm Hg, WBC count 9.2/µL, Hb 14 mg/dL, platelets 200,000/µL, sodium 137 mEq/L, potassium 5.0 mEq/L, chloride 96 mEq/L, BUN 28 mg/dL, creatinine 1.0 mg/dL, and glucose 120 mg/dL. Which of the following is the cause of this patient's acid-base abnormality?

Decreased oxygen delivery to tissues

Decreased ability for the tissues to use oxygen

Increased anions from toxic ingestion

Metabolic alkalosis mechanisms and compensation for USMLE Step 3: High-Yield Physiology Lessons

Pathophysiology - The Base Buildup

Definition: A primary ↑ in serum $HCO_3^−$, leading to ↑ pH.
Mechanism: Involves two distinct phases:
- Generation: The initial insult causing acid loss or base gain (e.g., vomiting, alkali ingestion).
- Maintenance: The kidney's failure to excrete the excess $HCO_3^−$. This is the key step and is primarily driven by:
  - Volume depletion (Euvolemia or hypovolemia)
  - Chloride depletion
  - Hypokalemia

Renal tubule bicarbonate reabsorption mechanisms

⭐ Contraction Alkalosis: Volume depletion → ↑ Angiotensin II → ↑ Proximal tubule $Na^+-H^+$ exchange → ↑ $HCO_3^−$ reabsorption, thus maintaining the alkalosis.

Etiologies - Acid Out, Bicarb In

H⁺ Loss (Acid Out):
- GI: Vomiting, nasogastric (NG) suction.
- Renal: Diuretics (loop/thiazide), hyperaldosteronism.
HCO₃⁻ Gain (Bicarb In):
- Exogenous: Citrate from blood transfusions, acetate in TPN, antacid abuse.

Category	Urine Chloride ($U_{Cl^−}$)	Key Causes
Saline-Responsive	< 20 mEq/L	- Vomiting / NG suction - Prior diuretic use
Saline-Resistant	> 20 mEq/L	- Hyperaldosteronism (Conn's) - Bartter & Gitelman syndromes

📌 Mnemonic: 'VOMIT' (Vomiting, Oversecretion of aldosterone, Mineralocorticoid excess, Iatrogenic, Total parenteral nutrition).

Compensation - The Body's Balancing Act

Respiratory Compensation: The lungs hypoventilate to retain acidic $CO_2$ and lower pH.
- Formula: Expected PaCO₂ = 0.7 * [HCO₃⁻] + 20 (± 5)
Renal Compensation: Kidneys try to excrete excess $HCO_3^−$. This is often impaired by volume depletion or hypokalemia, which maintains the alkalosis.
Paradoxical Aciduria: Despite systemic alkalosis, urine becomes acidic. This occurs in volume-depleted states (e.g., vomiting).

⭐ A low urine chloride (< 20 mEq/L) strongly suggests a saline-responsive metabolic alkalosis, where correcting the volume deficit is the primary treatment.

Renal tubule ion exchange in metabolic alkalosis

Metabolic alkalosis is driven by a primary ↑ in serum HCO₃⁻.

Differentiated by urine chloride (UCl⁻) and response to saline.

Saline-responsive (UCl⁻ < 20 mEq/L) is caused by volume & Cl⁻ depletion (e.g., vomiting, diuretics) and corrects with IV saline.

Saline-resistant (UCl⁻ > 20 mEq/L) is due to mineralocorticoid excess (e.g., Conn's, Cushing's).

Respiratory compensation involves hypoventilation, leading to a predictable ↑ in PaCO₂.

Often associated with hypokalemia due to intracellular K⁺ shifting.

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