1. Answer C. (MN-64) The patient presents with a high degree of suspicion for acute bacterial meningitis. Begin dexamethasone and antimicrobial therapy while obtaining blood cultures, as these are time-critical empiric treatments. Then initiate the diagnostic workup for bacterial meningitis by proceeding to an urgent head CT, followed by lumbar puncture if the scan is negative.

2. Answer E. (MN-64) Add ampicillin to the combination of a third- or fourth-generation cephalosporin and vancomycin given the patient’s age, to cover for Listeria monocytogenes. In addition to antibiotics, acyclovir should be started to cover for herpes simplex encephalitis as this disease is in the differential diagnosis.

3. Answer A. (MN-65) Central restricted diffusion due to central purulent material is the most specific MRI feature to help distinguish bacterial abscess from the majority of brain tumors. Other features, including location at the gray-white junction, central T2/FLAIR hyperintense signal, and rim enhancement are not specific and may occur in abscess, tumor, demyelinating diseases, or subacute stroke. The specificity of T2/FLAIR hypointense signal in the rim of the lesion has not been studied.

4. Answer C. (MN-65) The location in the frontal lobe and the solitary nature of the abscess make direct spread from frontal sinusitis the most likely source. Diabetes is a significant risk factor for epidural abscess but in the absence of sinus infection and if well-controlled at baseline is an uncommon isolated caused of brain abscess. Middle ear infection may extend into the adjacent temporal lobe or cerebellum to cause brain abscess. Endocarditis and pulmonary arteriovenous malformation may lead to hematogenous spread of bacteria (septic emboli), usually resulting in multiple brain abscesses in the distribution of the middle cerebral artery.

5. Answer E. (MN-65) Persistent enhancement of a brain abscess is common even with adequate treatment and should not be used in isolation to guide continuation of antibiotics if other clinical and radiographic features are reassuring. The size of the lesion should decrease, and the central restricted diffusion (corresponding to pus) should also decrease. Restricted diffusion in the ventricle may indicate ventriculitis, which is usually associated with worsening symptoms due to increased intracranial pressure. New areas of abscess formation should raise concern for additional septic emboli.

6. Answer E. (MN-65) Unlike other bacterial infections of the CNS, subdural empyema usually occurs in otherwise healthy patients. Paranasal sinusitis is the most common source of infection, with spread through venous sinuses to the subdural space. Diabetes and HIV may predispose to other CNS infections but are not specific risk factors for subdural empyema. Intravenous drug use and endocarditis are risk factors for multiple brain abscesses.

7. Answer D. (MN-65) The degree of neurological deficit prior to treatment remains the best predictor of outcome, so preventing further cord injury by emergent surgical intervention is essential. Age is a risk factor for both epidural abscess and worse outcome but is not modifiable. Alcohol dependence is a risk factor for epidural abscess but once infection occurs does not independently affect outcome. Corticosteroids, while beneficial in bacterial meningitis due to Streptococcus pneumoniae, are not indicated in spinal epidural abscess but are sometimes used nonetheless. All recommended empiric antibiotic regimens include agents and dosing with adequate CNS penetration.

8. Answer A. (MN-66) This patient most likely has an early but rapidly worsening case of Rocky Mountain spotted fever (RMSF). The possible mosquito bite was more likely a tick bite, which introduced Rickettsia rickettsii into her blood stream. The patient should be started immediately on empiric treatment with doxycycline, the first-line treatment for nonpregnant adults. Given the patient has early signs of encephalopathy, she should be admitted for close observation and is at high risk of further neurologic deterioration. An MRI brain may be useful but is not the first priority. Chloramphenicol may be used in the treatment of RMSF; however, it is second line and typically used when doxycycline is contraindicated. Elevated transaminase levels and organomegaly are more commonly seen in other rickettsial diseases, specifically ehrlichiosis and anaplasmosis; these tests are typically normal in the early phases of RMSF. Vancomycin and piperacillin are not effective in the treatment of rickettsial disease.

9. Answer E. (MN-66) Rocky Mountain spotted fever in moderate to severe cases results in cerebral vasculitic changes and scattered infarction; on contrast-enhanced MRI this appears as punctate lesions on T2 and DWI sequences.

10. Answer E. (MN-66) Leptospirosis most commonly causes a meningitis syndrome but can also manifest in the CNS as myelopathy, acute inflammatory demyelinating polyneuropathy, encephalitis, or cranial polyneuropathy. It is caused by the obligatory aerobic spirochete Leptospira and is vectored via rodents, particularly rats. Vectors for the other organisms include sheep, cattle, and goats (for Coxiella, Brucellosis, and Bacillus anthracis); pigs (for Brucellosis); and cats (for Bartonella henselae).

11. Answer B. (MN-66) The patient very likely has Bartonella henselae infection, which causes cat scratch disease. A probable diagnosis can be made on history and examination alone; however, generally it should be confirmed with ELISA-based serologic testing, which is the most sensitive and specific routine confirmatory test.

12. Answer D. (MN-67) The patient has a diagnosis of probable meningovascular neurosyphilis presenting with a stroke. Diagnostic criteria for neurosyphilis requires a serum positive treponemal and nontreponemal test and an elevated CSF protein or CSF WBC. CSF VDRL is a specific assay but carries a sensitivity of only 70%; therefore, a negative CSF VDRL does not rule out neurosyphilis if all other criteria are met. Untreated HIV-1 infection can cause a CSF pleocytosis, but a WBC >20 cells/mm³ should raise suspicion for a dual infection, including neurosyphilis, as neurosyphilis is 10 times more likely in HIV-1-infected patients. GeneXpert MTB/RIF PCR is a polymerase chain reaction test that can diagnose the presence of Mycobacterium tuberculosis and rifampin resistance within 2 hours.