WBR0383

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D. and Alison Leibowitz [1])]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Renal
Prompt	[[Prompt::A 54-year-old man is brought to the emergency department for confusion and altered consciousness. Appropriate history could not be taken due to the patient's clinical status. The patient’s arterial blood gas (ABG) and blood lab tests reveal the values displayed in the table below. What is the most likely underlying etiology of this patient's condition? ]]
Answer A	AnswerA::Narcotics abuse
Answer A Explanation	[[AnswerAExp::Narcotic abuse can cause hypoventilation. A respiratory acidosis with elevated pCO2 is characteristic among patients with narcotics abuse.]]
Answer B	AnswerB::Methanol intoxication
Answer B Explanation	[[AnswerBExp::Methanol intoxication can cause a high anion metabolic acidosis.]]
Answer C	AnswerC::Renal tubular acidosis
Answer C Explanation	[[AnswerCExp::Renal tubular acidosis manifests with a normal anion gap metabolic acidosis.]]
Answer D	AnswerD::Acetazolamide intoxication
Answer D Explanation	[[AnswerDExp::Acetazolamide intoxication typically causes a normal anion gap metabolic acidosis. Acetazolamide is often used to treat metabolic alkalosis.]]
Answer E	AnswerE::Severe vomiting
Answer E Explanation	[[AnswerEExp::Severe vomiting typically causes metabolic alkalosis due to loss of gastric. In vomiting, pH would have been > 7.4 with elevated bicarbonate concentration.]]
Right Answer	RightAnswer::B
Explanation	[[Explanation::The patient has a high anion-gap metabolic acidosis. The approach to acid-base disturbances begins with assessment of blood pH. Since the patient's blood pH is 7.26, he is in a state of acidosis. Because acidosis can be metabolic or respiratory, bicarbonate concentration and pCO2 are helpful in distinguishing between the two. In this scenario, the patient has metabolic acidosis, manifesting with low bicarbonate levels. Had this patient had respiratory acidosis, his pCO2 would have been elevated, which is not the case. There are two types of metabolic acidosis: Normal anion gap (hyperchloremic) and high anion gap (normochloremic). To differentiate between them, the equation Anion Gap = Sodium - (Chloride + Bicarbonate) is used. A normal anion-gap ranges btween 8-12 mEq/L. Identification of the type of acid-base abnormality is crucial because it aids in the diagnosis. The patient in this scenario has a high anion gap metabolic acidosis. In this case, anion gap = 140 - (100 + 12) = 30 mEq/L. Among the list of options, only methanol intoxication manifests with a high anion gap metabolic acidosis. Other causes of high anion gap metabolic acidosis can be remembered with the mnemonic KARMEL: Ketoacidosis, Aspirin intoxication, Renal failure, Methanol, Ethanol, Lactic acidosis. Educational Objective: Measurement of anion-gap in metabolic acidosis is essential to identify the likely diagnosis. Anion Gap = Sodium - (Chloride + Bicarbonate). A normal anion-gap ranges between 8-12 mEq/L. Common causes of high anion gap metabolic acidosis are KARMEL = Ketoacidosis - Aspirin toxicity – Renal failure – Methanol intoxication – Ethanol intoxication – Lactic acidosis References: First Aid 2014 page 328]]
Approved	Approved::Yes
Keyword	WBRKeyword::Methanol intoxication, WBRKeyword::Anion gap, WBRKeyword::High anion gap metabolic acidosis, WBRKeyword::Metabolic acidosis, WBRKeyword::Acidosis, WBRKeyword::Normochloremic metabolic acidosis, WBRKeyword::Hyperchloremic metabolic acidosis
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