**Question:** A 55-year-old woman has had several weeks of fever, abdominal pain, weight loss, and lack of energy. Three days prior to the assessment, she developed a left foot drop. Her blood pressure is 160/90 mm Hg, pulse 80/min, and physical examination confirms left peroneal nerve damage and a bilateral sensory peripheral neuropathy in both legs. There are no skin rashes. Laboratory evaluation reveals ESR of 105 mm/h, WBC of 14,000/mL, and negative serologic tests for antineutrophil cytoplasmic antibody (ANCA) and ANA. The eosinophil count is normal, and urinalysis is negative for casts, protein, and red cells. Which of the following is the most likely mechanism for renal injury in this condition?

A. Vasculitis
B. Vasculitis with eosinophilia
C. Polyarteritis nodosa
D. Polyarteritis nodosa with renal involvement

**Correct Answer:** D. Polyarteritis nodosa with renal involvement

**Core Concept:**

In this scenario, the patient presents with a combination of clinical symptoms, laboratory findings, and a negative ANCA and ANA tests, which are commonly associated with vasculitis. Vasculitis is an inflammation of blood vessels, leading to damage and potential organ involvement.

**Why the Correct Answer is Right:**

The correct answer, "Polyarteritis nodosa with renal involvement" (D), is chosen because it is a type of vasculitis characterized by inflammation of small- to medium-sized arteries and arterioles, including the renal arteries and arterioles, leading to renal involvement.

**Why Other Options are Incorrect:**

A. Vasculitis (without specifying renal involvement) is a broad term that includes various types of vasculitis but does not specifically address the renal involvement in this case.

B. Eosinophilia is not mentioned in this case, making this option incorrect. Eosinophilia is a clinical feature of eosinophilic granulomatosis with polyangiitis (EGPA), which is a distinct entity from polyarteritis nodosa.

C. Polyarteritis nodosa is a correct term, but without specifying renal involvement, it does not address the organ-specific manifestation in this case.

**Clinical Pearl:**

In cases like this, a high index of suspicion for vasculitis is crucial, particularly when assessing patients with systemic symptoms and signs, such as those described (fever, abdominal pain, weight loss, left foot drop, and elevated ESR), as well as renal involvement. A negative ANCA test is not a definitive test and may not rule out vasculitis in this context. The presence of a clinical picture consistent with vasculitis, such as the left foot drop, is more reliable than a negative ANCA test.

**Need for a Clinical Correlation:**

Renal involvement in vasculitis is a critical aspect to consider when evaluating patients with systemic symptoms and signs. A negative ANCA test does not rule out vasculitis, as ANCA tests may not be conclusive in all cases. The clinical picture including