WBR0434

Author	[[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::Neurology
Prompt	[[Prompt::A 28-year-old HIV-positive female presents to the emergency room with a high grade fever, headache, weakness, and tonic-clonic seizures. The patient has a recent history of anorexia. Laboratory work-up reveals CD4 count = 80 cells/µL per year. Head computer tomography (CT) scan demostrates multiple ring enhancing lesions in the frontal lobes. The patient requires treatment with a medication that uses 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 frequently 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 the 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, caused by Toxoplasma gondii, is an opportunistic parasitic infection that frequently infects HIV-positive patients with CD4 counts < 100 cells/µL per year. Toxoplasmosis manifest with fever, headache, and new-onset seizures. Multiple ring-enhancing lesions upon cranial 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, frequently occurring in HIV-positive individuals with CD4 count < 100 cells/µL per year, often manifests with a fever, headache, and new-onset seizures. Treatment of cerebral toxoplasmosis includes pyrimethamine, a dihydrofolate reductase inhibitor, and sulfadiazine, a sulfa drug that is a dihydropteroate synthetase inhibitor. References: First Aid 2014 page 151]]
Approved	Approved::Yes
Keyword	WBRKeyword::Toxoplasma gondii, WBRKeyword::toxomplasmosis, WBRKeyword::HIV, WBRKeyword::seizure, WBRKeyword::ring enhancing lesion, WBRKeyword::pyrimethamine, WBRKeyword::dihydrofolate reductase inhibitor
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