<s>[INST] context: Leptospirosis is a zoonosis of worldwide distribution. Human infection results from ingestion of water or food contaminated with leptospirae. Rats, mice, wild rodents, dogs, swine, and cattle excrete the organisms in urine and feces during active illness and during an asymptomatic carrier state. Drinking, swimming, bathing, or food consumption may lead to human infection. Children acquire the disease from dogs more often than do adults. Treatment can include doxycycline, ampicillin, or amoxicillin. Symptoms in humans range from fever and rash to jaundice through aseptic meningitis.Leptospirae are tightly coiled, thin, flexible spirochetes 5 to 15mm long, with one end bent into a hook. It stains best where silver can be impregnated into the organism. The gram stain would not be useful. The organisms derive energy from oxidation of fatty acids, so EMB or TM agar would be inadequate. Agglutination testing and CIE is complicated by the fact that all Leptospirae strains exhibit cross-reactivity in serologic tests. [/INST] question: A 12-year-old boy has sudden onset of fever, headache, and stiff neck. Two days earlier, he swam in a lake that is believed to have been contaminated with dog excreta. Leptospirosis is suspected. Which of the following laboratory tests is most appropriate to determine whether he has been infected with leptospira? | options: A: Agglutination test for leptospiral antigen B: Counter immune-electrophoresis of urine sample C: Gram stain of urine specimen D: Spinal fluid for dark-field microscopy and culture in Fletcher serum medium | answer: D </s>