**Core Concept**
Jarisch-Herxheimer reaction is an acute febrile illness that occurs in some patients after the initiation of antibiotic therapy for syphilis, particularly after the administration of penicillin. This reaction is caused by the rapid release of endotoxins from the spirochetes, leading to a systemic inflammatory response.

**Why the Correct Answer is Right**
The Jarisch-Herxheimer reaction is a classic example of an immune-mediated reaction, where the body's immune response to the dying bacteria leads to the release of inflammatory mediators, such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1β), and interleukin-6 (IL-6). These cytokines cause the symptoms of the reaction, including fever, chills, headache, and myalgias. The reaction typically occurs within the first 24 hours of antibiotic therapy and resolves within 24-48 hours.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the Jarisch-Herxheimer reaction. While it is true that some patients with syphilis may experience an allergic reaction to penicillin, this is not the same as the Jarisch-Herxheimer reaction.
**Option B:** This option is incorrect because the Jarisch-Herxheimer reaction is not a common side effect of other antibiotics, such as doxycycline or ceftriaxone.
**Option C:** This option is not accurate because the Jarisch-Herxheimer reaction is not a common side effect of other bacterial infections, such as tuberculosis or pneumonia.

**Clinical Pearl / High-Yield Fact**
To differentiate the Jarisch-Herxheimer reaction from other acute illnesses, clinicians should consider the patient's recent antibiotic therapy and history of syphilis. The reaction typically occurs within the first 24 hours of therapy and resolves within 24-48 hours.

**Correct Answer: C. Syphilis.**