WBR1107

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pathology, MainCategory::Pathophysiology, MainCategory::Pharmacology
Sub Category	SubCategory::Hematology, SubCategory::Pulmonology
Prompt	[[Prompt::A 35-year old man with HIV and documented trimethoprim-sulfamethoxazole allergy is placed on dapsone for pneumocystis prophylaxis after his CD4 count drops below 200/mm3. 3 weeks later, the patient presents to the emergency room with a 12-hour history of dyspnea, headache, dizziness and blue discoloration of the lips and extremities. Pulse oximetry reveals an oxygen saturation of 82% on room air. When blood is drawn for arterial-blood gas measurement, the nurse notices that it has a chocolate-brown color. Which of the following would be the most effective pharmacotherapy?]]
Answer A	AnswerA::Methylene blue
Answer A Explanation	AnswerAExp::Methylene blue can be used to reverse methemoglobinemia.
Answer B	AnswerB::Thiosulfate
Answer B Explanation	AnswerBExp::Thiosulfate can be used to reverse methomglobinemia when cyanide poisoning is the cause. However, in this case Dapsone was responsible for this patient’s methomglobinemia.
Answer C	AnswerC::N-acetyl-cysteine
Answer C Explanation	AnswerCExp::N-acetyl-cysteine is used to reverse the toxicity of acetaminophen overdose.
Answer D	AnswerD::Naloxone
Answer D Explanation	AnswerDExp::Naloxone is used to reverse the effects of opiate overdose.
Answer E	AnswerE::Vitamin C
Answer E Explanation	AnswerEExp::Vitamin C can be effective in treating methemoglobinemia, but it is considered an ancillary therapy.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Methemoglobinemia is a disorder characterized by the presence of a higher than normal level of methemoglobin (metHb, i.e., ferric [Fe3+] rather than ferrous [Fe2+] haemoglobin) in the blood. Methemoglobin is a form of hemoglobin that contains ferric [Fe3+] iron and has a decreased ability to bind oxygen. Methemoglobinemia can be treated with supplemental oxygen and methylene blue 1% solution (10 mg/ml) 1 to 2 mg/kg administered intravenously slowly over five minutes followed by IV flush with normal saline. Methylene blue restores the iron in hemoglobin to its normal (reduced) oxygen-carrying state. This is achieved by providing an artificial electron acceptor (such as methylene blue, or flavin) for NADPH methemoglobin reductase (RBCs usually don't have one; the presence of methylene blue allows the enzyme to function at 5x normal levels) The NADPH is generated via the hexose monophosphate shunt. Educational Objective: Methemoglobinemia is most effectively treated with methylene blue. References: First Aid 2015 page 604 First Aid 2014 page 597]]
Approved	Approved::Yes
Keyword	WBRKeyword::Methemoglobinemia, WBRKeyword::Cyanosis, WBRKeyword::Blood, WBRKeyword::Oxygen, WBRKeyword::Pulmonary, WBRKeyword::Breathing, WBRKeyword::Lung, WBRKeyword::Hemoglobin
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