WBR0434

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 28 year old HIV-positive female patient presents to the emergency department with high grade fever, headache, and new onset tonic-clonic seizures. The patient also explains she has been anorexic and very weak recently. Laboratory work-up reveals CD4 count = 80 cells/µL per year. Head computer tomography (CT) scan shows multiple ring enhancing lesions in the frontal lobes. The patient requires treatment with a medication that has which of the following mechanisms of action?]]
Answer A	AnswerA::Inhibition of heme polymerase activity
Answer A Explanation	AnswerAExp::Chloroquine, an antimalarial drug, is a plasmodium heme polymerase inhibitor.
Answer B	AnswerB::Inhibition of dihydrofolate reductase activity
Answer B Explanation	AnswerBExp::Pyrimethamine, a drug used to treat toxoplasmosis, is a dihydrofolate reductase inhibitor.
Answer C	AnswerC::Free radical toxicity of organism’s DNA
Answer C Explanation	AnswerCExp::Metronidazole, an antibiotic and antiprotozoal, forms free radical metabolites that are toxic to bacterial DNA
Answer D	AnswerD::Blockade of peptide bond formation at 50S ribosomal subunit
Answer D Explanation	AnswerDExp::Several antibiotics, including chloramphenicol, macrolides, clindamycin, streptogramins, and linezolid, block peptide bond formation at 50S ribosomal subunit.
Answer E	AnswerE::Inhibition of DNA polymerase activity
Answer E Explanation	AnswerEExp::Antiviral medications, such as acyclovir, ganciclovir, foscarnet, and cidofovir are DNA polymerase inhibitors.
Right Answer	RightAnswer::B
Explanation	[[Explanation::Cerebral toxoplasmosis is caused by Toxoplasma gondii, an opportunistic parasitic infection that often infects HIV-positive patients with CD4 counts < 100 cells/µL per year. Toxoplasmosis causes a clinical syndrome of fever, headache, and new-onset seizures typically in an HIV-positive patient. Multiple ring-enhancing lesions on head CT scan are characteristic. Treatment of cerebral toxoplasmosis includes pyrimethamine, a dihydrofolate reductase inhibitor, and sulfadiazine, a sulfa drug that is a dihydropteroate synthetase inhibitor. Educational Objective: Cerebral toxoplasmosis causes a clinical syndrome of fever, headache, and new-onset seizures typically in an HIV-positive patient with CD4 count < 100 cells/µL per year. Treatment of cerebral toxoplasmosis includes pyrimethamine, a dihydrofolate reductase inhibitor, and sulfadiazine, a sulfa drug that is a dihydropteroate synthetase inhibitor. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Toxoplasma gondii, WBRKeyword::toxomplasmosis, WBRKeyword::HIV, WBRKeyword::seizure, WBRKeyword::ring enhancing lesion, WBRKeyword::pyrimethamine, WBRKeyword::dihydrofolate reductase inhibitor
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