WBR0790

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pharmacology
Sub Category	SubCategory::Infectious Disease
Prompt	[[Prompt::A 52-year-old man previously diagnosed with AIDS with a recent CD4 count of 28 presents to the HIV clinic for 2 weeks of slowly decreasing visual acuity. Concerned about the possibility of infection, the infectious disease fellow admits him for treatment. One week into the treatment the patient starts experiencing anxiety, muscle twitching, and tremulousness. Blood tests reveal significant hypomagnesemia, hypocalcemia, and hypophosphatemia. Which of the following agents was the patient started on?]]
Answer A	AnswerA::Acyclovir
Answer A Explanation	AnswerAExp::Acyclovir is not active against CMV so it should never be used in suspected CMV retinitis. Acyclovir is not known to cause electrolyte imbalances.
Answer B	AnswerB::Valacyclovir
Answer B Explanation	AnswerBExp::Valacyclovir is a prodrug of acyclovir that has a better bioavailability than acyclovir. It is not active against CMV. Valacyclovir is not known to cause electrolyte imbalances.
Answer C	AnswerC::Amphotericin B
Answer C Explanation	AnswerCExp::Amphotericin B is an antifungal that has no use in CMV retinitis. It is however known to cause elctrolyte imbalances especially with the initial doses.
Answer D	AnswerD::Cidofovir
Answer D Explanation	AnswerDExp::Cidofovir is another agent used in the treatment of resistant CMV. It is known to cause nephrotoxicity but does not usually cause electrolyte imbalances.
Answer E	AnswerE::Foscarnet
Answer E Explanation	AnswerEExp::Foscarnet is used in the treatment of CMV retinitis and although not very common, can cause electrolyte imbalances typically hypomagnesemia, hypocalcemia, and hypophosphatemia.
Right Answer	RightAnswer::E
Explanation	[[Explanation::Foscarnet is an antiviral pyrophosphate analog that is predominantly used in patients with moderate to severe herpes virus and CMV infections. The patient is this scenario is presenting with decreasing visual acquity in the context of a very low CD4 count raising the suspicion for CMV retinitis. Foscarnet acts by binding proximally to the pyrophosphate-binding site on the viral DNA polymerase, blocking the exchange of pyrophosphate from deoxynucleotide triphosphates and leading to DNA chain termination. Forscarnet does not require any in vivo modification to be functional compared to other antivirals like acyclovir and ganciclovir. A rare but serious complication of foscarnet administration is electrolyte imbalance particularly hypomagnesemia, hypocalcemia, and hypophosphatemia. It is common to observe a decrease in PTH levels as well. This patient has the typical presentation CMV retinitis which he is at high risk for given his CD4 count. Any patient with a CD4<50 and visual disturbances should be worked-up for CMV retinitis. Educational Objective: A rare complication of foscarnet is electrolyte imbalance, typically hypomagnesemia, hypocalcemia, and hypophosphatemia. References: Buckley MS, Leblanc JM, Cawley MJ. Electrolyte disturbances associated with commonly prescribed medications in the intensive care unit. Crit Care Med. 2010;38(6 Suppl):S253-64.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Foscarnet, WBRKeyword::Electrolyte imbalance, WBRKeyword::CMV retinitis, WBRKeyword::Antivirals, WBRKeyword::Side Effects
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