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Difficulty: MediumFormulating a Differential Diagnosis

A 42-year-old man presents to the clinic with a 6-week history of dry cough, low-grade fevers, fatigue, and unintended weight loss of 3 kg (6.6 lbs). He lives in Phoenix, Arizona, and works as an excavator operator for a commercial construction company. His medical history is unremarkable, and he takes no medications. On physical examination, his temperature is 37.8°C (100.0°F), blood pressure is 122/78 mmHg, pulse is 84/min, and respiratory rate is 16/min. Lung auscultation is clear bilaterally. Examination of the lower extremities reveals several tender, erythematous, subcutaneous nodules on the anterior shins. Laboratory studies show a leukocyte count of 8,500/mm³ with 9% eosinophils. Erythrocyte sedimentation rate is 65 mm/h. A chest radiograph demonstrates bilateral hilar lymphadenopathy and a thin-walled cavity in the right upper lobe. Which of the following is the most likely diagnosis?

  1. CoccidioidomycosisAnswer
  2. B
    Sarcoidosis
  3. C
    Mycobacterium tuberculosis infection
  4. D
    Histoplasmosis
  5. E
    Granulomatosis with polyangiitis

Answer

Coccidioidomycosis
The correct diagnosis is Coccidioidomycosis. This patient's presentation of subacute pulmonary symptoms, erythema nodosum (often referred to clinically as 'Desert rheumatism' when associated with this infection), eosinophilia, and a thin-walled cavity on chest imaging is highly characteristic of primary pulmonary coccidioidomycosis. The diagnosis is strongly supported by his geographic location (Phoenix, Arizona) and occupational exposure to aerosolized soil (excavator operator).

Step-by-Step Solution

1
Analyze the patient's demographics, occupation, and geographic location.
The patient is a 42-year-old excavator operator living in Phoenix, Arizona, indicating significant exposure to disturbed arid soil in the Southwestern United States.
Geographic history is critical for differentiating endemic fungal infections.
2
Evaluate the clinical symptoms and physical examination findings.
The patient has subacute constitutional symptoms (fever, weight loss), pulmonary symptoms (cough), and tender nodules on the shins (erythema nodosum).
Erythema nodosum combined with pulmonary symptoms often narrows the differential to systemic inflammatory or specific infectious etiologies.
3
Synthesize the objective laboratory and imaging data.
Labs show eosinophilia (9%) and elevated ESR. Chest radiography shows bilateral hilar lymphadenopathy and a thin-walled right upper lobe cavity.
Eosinophilia paired with cavitary lung disease strongly suggests a fungal or parasitic etiology rather than a purely autoimmune or mycobacterial process.
4
Compare findings against the differential diagnoses.
The combination of Arizona soil exposure, erythema nodosum ('Desert rheumatism'), eosinophilia, and cavitary lung disease perfectly matches Coccidioidomycosis, ruling out Sarcoidosis (lacks cavity/eosinophilia) and TB (lacks prominent eosinophilia/erythema nodosum).
Accurate differential diagnosis requires integrating all atypical and characteristic findings to avoid premature closure on a single symptom cluster.

Key Concept

Formulating a differential diagnosis for subacute cavitary lung disease by integrating geographic exposure, cutaneous manifestations (erythema nodosum), and specific laboratory findings (eosinophilia).
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Formulating a Differential Diagnosis

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