A 9-year-old child is brought to the emergency room with the chief complaint of enlarged, painful axillary lymph nodes. The resident physician also notes a small, inflamed, dime-sized lesion surrounding what appears to be a small scratch on the forearm. The lymph node is aspirated and some pus is sent to the laboratory for examination. A Wahin-Starry silver impregnation stain reveals many highly pleomorphic, rod-shaped bacteria. The most likely cause of this infection is

**Core Concept**
The underlying principle in this question is the identification of a specific type of bacterial infection that causes lymphadenopathy and cellulitis, particularly in children. The key is recognizing the distinctive morphology of the bacteria, which is crucial for accurate diagnosis and treatment.

**Why the Correct Answer is Right**
The presence of highly pleomorphic, rod-shaped bacteria on a Wahin-Starry silver impregnation stain is characteristic of *Bartonella henselae*, the causative agent of cat-scratch disease (CSD). This bacterium is typically transmitted through scratches or bites from infected cats, leading to local inflammation and subsequent lymphadenopathy. The clinical presentation of a small, inflamed lesion at the site of the scratch, accompanied by regional lymphadenopathy, is a classic feature of CSD. The bacteria's rod-shaped morphology and pleomorphic nature are hallmarks of *Bartonella* species.

**Why Each Wrong Option is Incorrect**
**Option A:** *Staphylococcus aureus* is a common cause of skin and soft tissue infections, but it does not typically present with the distinctive rod-shaped morphology seen on the stain. While *S. aureus* can cause lymphadenopathy, the clinical presentation and bacteriological findings in this case do not align with this diagnosis.
**Option B:** *Klebsiella pneumoniae* is a gram-negative rod, but it does not match the description of the bacteria seen on the stain. Furthermore, *K. pneumoniae* is more commonly associated with urinary tract infections, pneumonia, and other systemic infections, rather than localized infections like CSD.
**Option C:** *Francisella tularensis* is the causative agent of tularemia, which can present with lymphadenopathy and skin lesions. However, the clinical presentation and bacteriological findings in this case do not align with tularemia, as the stain reveals rod-shaped bacteria rather than the characteristic bipolar staining pattern of *F. tularensis*.

**Clinical Pearl / High-Yield Fact**
In children presenting with enlarged, painful lymph nodes and a history of a scratch or bite from an animal, cat-scratch disease should be a high consideration in the differential diagnosis. Remember the classic clinical presentation of CSD: a small, inflamed lesion at the site of the scratch, accompanied by regional lymphadenopathy.

**Correct Answer:** D. *Bartonella henselae*.

✓ Correct Answer: D. Baonella henselae