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::'''Correct:''' Methylene blue can be used to reverse methemoglobinemia. |
| Answer B | AnswerB::Thiosulfate |
| Answer B Explanation | AnswerBExp::'''Incorrect:''' 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::'''Incorrect:''' N-acetyl-cysteine is used to reverse the toxicity of acetaminophen overdose. |
| Answer D | AnswerD::Naloxone |
| Answer D Explanation | AnswerDExp::'''Incorrect:''' Naloxone is used to reverse the effects of opiate overdose. |
| Answer E | AnswerE::Vitamin C |
| Answer E Explanation | AnswerEExp::'''Incorrect:''' 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 2014 page 597 |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Methemoglobinemia, WBRKeyword::Cyanosis, WBRKeyword::Blood, WBRKeyword::Oxygen, WBRKeyword::Pulmonary, WBRKeyword::Breathing, WBRKeyword::Lung, WBRKeyword::Hemoglobin |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |