Rocky Mountain spotted fever is caused by Rickettsia rickettsii, an obligate intracellular, gram-negative bacterium in the spotted fever group. It is transmitted to humans by ticks, most commonly Dermacentor species. The organism targets endothelial cells, causing vasculitis that leads to the abrupt fever, severe headache, and the characteristic rash that begins on the wrists and ankles and spreads inward. Early recognition and treatment with doxycycline are important for a good outcome. The other organisms listed cause different diseases: Treponema pallidum causes syphilis, Borrelia burgdorferi causes Lyme disease, and Bordetella pertussis causes whooping cough. None of these are the etiologic agent of Rocky Mountain spotted fever.

Pain out of proportion to what you can see on exam is the strongest clue for Acanthamoeba keratitis. The organism tends to invade corneal nerves and deeper tissues, so patients often experience severe, disabling pain that seems disproportionate to only mild surface findings early on. In bacterial keratitis, pain typically correlates with visible surface damage and infection signs like epithelial loss, discharge, and redness. Ring infiltrates or other later signs can appear with Acanthamoeba, but they aren’t as reliable early indicators. A rapid improvement with antibiotics would argue against Acanthamoeba, so the standout feature is the extreme pain relative to minimal corneal findings.

Acanthamoeba grows best on non-nutrient agar that is overlaid with heat-killed E. coli because the amoebae feed on bacteria, providing a simple, controlled food source without allowing bacterial overgrowth. The non-nutrient surface itself doesn’t supply nutrients, so the only growth cue comes from the bacteria the amoebae consume, allowing trophozoites and cysts to develop for observation. Other media are designed to culture bacteria or fungi and don’t offer this bacterial food source on a minimal surface, which is why they aren’t used for growing Acanthamoeba.

Toxic shock syndrome is most strongly linked to a bacterium that is coagulase positive and produces a potent superantigen toxin. Staphylococcus aureus fits this description because it makes TSST-1, a toxin that acts as a superantigen and triggers a massive release of cytokines, leading to fever, rash, and shock. The coagulase test distinguishes this organism from other common Gram-positive cocci. Staphylococcus epidermidis is coagulase negative and typically causes device-related or opportunistic infections rather than toxic shock. Streptococcus pyogenes can cause a toxic shock–like syndrome, but it is not coagulase positive, and the classic toxic shock mechanism differs. Streptococcus pneumoniae is not associated with toxic shock and is not coagulase positive. So the organism that accounts for both toxic shock syndrome and coagulase positivity is Staphylococcus aureus.