This patient has symptoms consistent with long-standing Lyme disease.  Lyme disease is caused by the spirochete Borrelia burgdorferi, which is transmitted by the bite of an Ixodes tick.  Lyme disease is endemic in the northeastern United States and northern Europe.  Like syphilis, another spirochetal infection, Lyme disease progresses through a predictable series of phases:

1. Early localized phase: Days to weeks following exposure, patients experience flu-like symptoms and the characteristic cutaneous eruption, erythema chronicum migrans (ECM).
2. Early disseminated phase: Weeks or months later, there may be central nervous system (eg, facial palsy) and/or cardiac involvement (eg, atrioventricular nodal block).
3. Late Lyme disease: This phase, which occurs months to years post-exposure, is rare given the frequency with which patients receive antibiotics for other indications.  Patients may experience asymmetric arthritis (most often involving a single knee joint, as in this patient) and/or subacute encephalopathy with decreased memory, somnolence, and mood changes.

Lyme disease is easily treated with doxycycline or penicillin-type antibiotics (eg, ceftriaxone).  This patient does not appear to have noted ECM, for which he would likely have received doxycycline.  However, treatment with ceftriaxone when he developed the facial palsy (a possible manifestation of early disseminated Lyme disease) would have prevented progression to late Lyme disease.

(Choice A)  Allopurinol competitively inhibits xanthine oxidase, suppressing uric acid synthesis.  This drug is used to treat gout and prevent uric acid kidney stones and tumor lysis syndrome.

(Choice C)  Lamivudine is a nucleoside reverse transcriptase inhibitor used for HIV treatment.

(Choice D)  A Borrelia burgdorferi vaccine, now discontinued due to low sales, contains a recombinant outer surface protein from these bacteria, not live organisms.

(Choice E)  Mefloquine is an antimalarial drug used in prevention and treatment of Plasmodium falciparum infections, among others.

(Choice F)  Metronidazole is effective against anaerobic bacteria (eg, Clostridium difficile, Bacteroides) and parasites (eg, Entamoeba histolytica, Giardia lamblia, Trichomonas vaginalis).

Educational objective:
Early Lyme disease causes flu-like symptoms and erythema chronicum migrans.  The second stage of Lyme disease may involve atrioventricular block and facial palsy.  Late Lyme disease can cause chronic asymmetric large joint arthritis and encephalopathy.  Lyme disease is easily treated with doxycycline or penicillin-type antibiotics (eg, ceftriaxone).