A 58-year-old man with acute leukemia undergoing induction chemotherapy develops persistent fever (38.8°C) unresponsive to broad-spectrum antibiotics for 5 days. He is neutropenic (ANC 200/μL). Blood cultures remain negative. Chest X-ray shows multiple nodular infiltrates. High-resolution CT chest reveals the 'halo sign.' Serum β-D-glucan is markedly elevated (>500 pg/mL). Empirical antifungal therapy is initiated. Which Candida species is most likely responsible for this invasive infection?

Explanation

## Invasive Candidiasis in Severe Immunosuppression ### Clinical Context **Key Point:** This patient has severe neutropenia (ANC <500/μL) from chemotherapy — a major risk factor for invasive fungal infection. The 'halo sign' on CT (ground-glass opacification surrounding a nodule) is **classically associated with invasive pulmonary aspergillosis** in neutropenic patients, but the question asks specifically about the most likely *Candida* species responsible for invasive candidiasis in this setting. ### Why Candida albicans? **High-Yield:** *Candida albicans* remains the **most common cause of invasive candidiasis overall**, accounting for approximately 40–50% of all cases. In neutropenic patients undergoing induction chemotherapy for acute leukemia: 1. **Most frequent species** — *C. albicans* is the leading cause of invasive candidiasis across all immunocompromised populations (Harrison's Principles of Internal Medicine, 21st ed.) 2. **Pulmonary involvement** — *C. albicans* can cause hematogenously disseminated disease with pulmonary nodules in profoundly neutropenic hosts 3. **Negative blood cultures** — do NOT exclude invasive candidiasis; sensitivity of blood cultures for deep-tissue candidiasis is only ~50% 4. **Elevated β-D-glucan (>500 pg/mL)** — consistent with invasive fungal disease; β-D-glucan is a cell wall component of *Candida* spp. and other fungi ### Epidemiology of Invasive Candidiasis by Species | Species | Frequency in Invasive Disease | Antifungal Susceptibility | Clinical Setting | |---------|-------------------------------|--------------------------|------------------| | **Candida albicans** | 40–50% (most common) | Fluconazole-susceptible | Both mucosal & invasive | | **Candida glabrata** | 15–25% (second most common) | Reduced fluconazole susceptibility | Invasive, bloodstream | | **Candida parapsilosis** | 10–15% | Fluconazole-susceptible | Catheter-associated | | **Candida auris** | Emerging | Multidrug-resistant | Healthcare-associated | ### Why NOT the Other Options? - **Candida glabrata (D):** Second most common cause of invasive candidiasis, but *C. albicans* remains more prevalent overall. *C. glabrata* is more prominent in patients on fluconazole prophylaxis (breakthrough infections), which is not specified here. - **Candida auris (A):** An emerging, multidrug-resistant pathogen associated with healthcare outbreaks; not the most common cause of invasive candidiasis. - **Candida parapsilosis (C):** Primarily associated with catheter-related bloodstream infections and neonatal candidiasis; less common in neutropenic leukemia patients. ### Diagnostic Clues in This Case - **Severe neutropenia** (ANC 200/μL) — major risk factor for invasive candidiasis - **Negative blood cultures** — does NOT exclude invasive candidiasis - **Nodular infiltrates / halo sign** — invasive fungal infection (classically Aspergillus, but Candida can also cause pulmonary nodules) - **Elevated β-D-glucan** — fungal cell wall marker, highly sensitive for invasive candidiasis - **Failure of broad-spectrum antibiotics** — strongly suggests fungal etiology **Clinical Pearl:** Per IDSA guidelines and Harrison's, *Candida albicans* is the single most common species causing invasive candidiasis in immunocompromised hosts. While non-*albicans* species are increasing in frequency, *C. albicans* still predominates when no specific prophylaxis history is given (Pappas et al., IDSA Guidelines 2016; Harrison's 21st ed., Chapter on Candidiasis).