WBR0383

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 54-year-old male is brought to the ER for confusion and altered consciousness. The patient’s arterial blood gas (ABG) and blood lab tests reveal the values as shown in the table below. Which of the following causes most likely led to this patient’s condition?]]
Answer A	AnswerA::Narcotics abuse
Answer A Explanation	AnswerAExp::Narcotic abuse causes hypoventilation. A respiratory acidosis with elevated pCO2 would have been present.
Answer B	AnswerB::Methanol intoxication
Answer B Explanation	AnswerBExp::Methanol intoxication can cause a high anion metabolic acidosis clinical picture.
Answer C	AnswerC::Renal tubular acidosis
Answer C Explanation	AnswerCExp::Renal tubular acidosis presents with a normal anion gap metabolic acidosis.
Answer D	AnswerD::Acetazolamide intoxication
Answer D Explanation	AnswerDExp::Acetazolamide intoxication causes a normal anion gap metabolic acidosis. Acetazolamide is used to treat metabolic alkalosis
Answer E	AnswerE::Severe vomiting
Answer E Explanation	[[AnswerEExp::Severe vomiting causes metabolic alkalosis due to loss of gastric acid while vomiting. pH would have been > 7.4 with elevated bicarbonate.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::The patient presents with a high anion-gap metabolic acidosis. The approach to acid-base disturbances starts with assessment of pH. Since pH = 7.26, the patient is in a state of acidosis. Since acidosis could either be metabolic or respiratory, bicarbonate and pCO2 are both helpful in the distinguishing between the two. In this vignette, the patient has metabolic acidosis, as demonstrated by low bicarbonate. Had the patient had respiratory acidosis, his pCO2 would have been elevated, which is not the case. There are 2 types of metabolic acidosis: Normal anion gap (hyperchloremic) or high anion gap (normochloremic). To differentiate them, the equation: Anion Gap = Sodium – (Chloride + Bicarbonate) is used. A normal anion-gap is in the range of 8-12 mEq/L. This case is an example of high anion gap metabolic acidosis. Anion gap = 140 – (100 + 12) = 30 mEq/L. Identification of the type of acid-base abnormality is sometimes crucial because it can be very helpful in the diagnosis. Among the list of options, only methanol intoxication reveals a high anion gap metabolic acidosis. Other causes of high anion gap metabolic acidosis can be remembered by KARMEL: Ketoacidosis – Aspirin intoxication - Renal failure – Methanol – Ethanol - Lactic acidosis. Educational Objective: Measurement of anion-gap in metabolic acidosis is important to identify possible diagnoses. Anion-gap = Sodium – (Chloride + Bicarbonate). Common causes of high anion gap metabolic acidosis are KARMEL = Ketoacidosis - Aspirin toxicity – Renal failure – Methanol intoxication – Ethanol intoxication – Lactic acidosis Educational Objective: References: ]]
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