WBR1107

Author	[[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	USMLE Step 1
Main Category	Pharmacology
Sub Category	Pulmonology
Prompt	A 35-year-old man with HIV and documented trimethoprim-sulfamethoxazole allergy is placed on dapsone for pneumocystis prophylaxis when his CD4 count dropped below 200/mm3. Three weeks later, the patient presents to the emergency department 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 to manage this patient's condition?
Answer A	Methylene blue
Answer A Explanation	Methylene blue reverses methemoglobinemia.
Answer B	Thiosulfate
Answer B Explanation	Thiosulfate reverses methomglobinemia in cases of cyanide poisoning. However, in this case dapsone pharmacotherapy was responsible for this patient’s methomglobinemia.
Answer C	N-acetylcysteine
Answer C Explanation	N-acetylcysteine reverses the toxicity of acetaminophen overdose.
Answer D	Naloxone
Answer D Explanation	Naloxone reverses the effects of opiate overdose.
Answer E	Vitamin C
Answer E Explanation	Vitamin C can be effective in treating methemoglobinemia, but it is considered an ancillary therapy.
Right Answer	A
Explanation	[[Explanation::Methemoglobinemia is a disorder characterized by the presence of a higher than normal concentration 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 (methylene blue allows the enzyme to be five-fold more active). The NADPH is generated via the hexose monophosphate shunt. Educational Objective: Methemoglobinemia can be treated with supplemental oxygen and methylene blue 1% solution. Methylene blue restores the iron in hemoglobin to its normal (reduced) oxygen-carrying state. References: First Aid 2015 page 604.]]
Approved	Yes
Keyword	Methemoglobinemia, Cyanosis, Blood, Oxygen, Pulmonary, Breathing, Lung, Hemoglobin
Linked Question
Order in Linked Questions