A clinically depressed farmer complains of extreme weakness, a daily rise and fall in fever, and night sweats. Small gram-negative rods are isolated from blood cultures after a 2-week incubation period. Which of the following organisms is the most likely etiologic agent?
**Core Concept**
The question requires the identification of an organism causing a specific clinical presentation in a patient with a history of depression. The patient's symptoms, such as fever, night sweats, and weakness, along with the isolation of small gram-negative rods from blood cultures, suggest an infection. The clinical context of depression and the time frame for incubation are crucial for narrowing down the possible causes.
**Why the Correct Answer is Right**
The organism causing this infection is likely *Brucella melitensis*, a gram-negative rod that can cause brucellosis. This zoonotic disease is transmitted to humans through contact with infected animals, such as sheep or goats, or through consuming contaminated dairy products. The incubation period of 2 weeks is consistent with brucellosis, which can present with systemic symptoms like fever, night sweats, and weakness. The connection between depression and brucellosis may seem unrelated, but depression can be a manifestation of the infection itself, as well as a consequence of chronic illness.
**Why Each Wrong Option is Incorrect**
* **Option A:** *Francisella tularensis* is another gram-negative rod that causes tularemia, typically acquired through contact with infected animals or contaminated water. While tularemia can present with fever and weakness, it is less commonly associated with night sweats and does not typically have a 2-week incubation period.
* **Option B:** *Yersinia enterocolitica* is a gram-negative rod that causes gastrointestinal infections, often presenting with diarrhea, abdominal pain, and fever. It is not typically associated with systemic symptoms like night sweats and weakness, nor is it commonly isolated from blood cultures.
* **Option C:** *Borrelia burgdorferi* is a spirochete that causes Lyme disease, acquired through tick bites. While Lyme disease can cause fever and weakness, it is not typically associated with night sweats or the isolation of small gram-negative rods from blood cultures.
**Clinical Pearl / High-Yield Fact**
Brucellosis is often referred to as the "undulant fever" due to its characteristic pattern of fever that waxes and wanes. This clinical presentation is a classic clue to the diagnosis of brucellosis, especially in patients with a history of exposure to infected animals or contaminated dairy products.
**Correct Answer:** C. *Brucella melitensis*