A 69-year-old man with metastatic colon cancer is brought to the emergency department because of shortness of breath, fever, chills, and a productive cough with streaks of blood for the past 5 days. He has a history of emphysema. The patient does not have abdominal pain or headache. He receives chemotherapy with 5-fluorouracil, leucovorin, and oxaliplatin every 6 weeks; his last cycle was 3 weeks ago. His temperature is 38.3°C (101°F), pulse is 112/min, and blood pressure is 100/70 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 83%. A few scattered inspiratory crackles are heard over the right lung. His mucous membranes are dry. Cardiac examination is normal. Laboratory studies show: Hemoglobin 9.3 mg/dL Leukocyte count 700/mm3 Segmented neutrophils 68% Lymphocytes 25% Eosinophils 4% Monocytes 3% Platelet count 104,000/mm3 Serum Glucose 75 mg/dL Urea nitrogen 41 mg/dL Creatinine 2.1 mg/dL Galactomannan antigen Positive Which of the following is the most appropriate initial pharmacotherapy?

Rifampin, isoniazid, pyrazinamide, and ethambutol

A 7-year-old boy with a history of cystic fibrosis is brought to the physician for evaluation of recurrent episodes of productive cough, wheezing, and shortness of breath over the past month. Physical examination shows coarse crackles and expiratory wheezing over both lung fields. Serum studies show elevated levels of IgE and eosinophilia. A CT scan of the lungs shows centrally dilated bronchi with thickened walls and peripheral airspace consolidation. Antibiotic therapy is initiated. One week later, the patient continues to show deterioration in lung function. A sputum culture is most likely to grow which of the following?

Monomorphic, septate hyphae that branch at acute angles

Monomorphic, broad, nonseptate hyphae that branch at wide angles

Dimorphic, broad-based budding yeast

Dimorphic, cigar-shaped budding yeast

Monomorphic, narrow budding encapsulated yeast

A 31-year-old female undergoing treatment for leukemia is found to have a frontal lobe abscess accompanied by paranasal swelling. She additionally complains of headache, facial pain, and nasal discharge. Biopsy of the infected tissue would most likely reveal which of the following?

Spherules containing endospores

Budding yeast with a narrow base

A 58-year-old man presents with a high-grade fever, throbbing left-sided headache, vision loss, and left orbital pain. He says that his symptoms started acutely 2 days ago with painful left-sided mid-facial swelling and a rash, which progressively worsened. Today, he woke up with complete vision loss in his left eye. His past medical history is significant for type 2 diabetes mellitus, diagnosed 5 years ago. He was started on an oral hypoglycemic agent which he discontinued after a year. His temperature is 38.9°C (102.0°F), blood pressure is 120/80 mm Hg, pulse is 120/min, and respiratory rate is 20/min. On examination, there is purulent discharge from the left eye and swelling of the left half of his face including the orbit. Oral examination reveals extensive necrosis of the palate with a black necrotic eschar and purulent discharge. Ophthalmic examination is significant for left-sided ptosis, proptosis, and an absence of the pupillary light reflex. Laboratory findings are significant for a blood glucose level of 388 mg/dL and a white blood cell count of 19,000 cells/mm³. Urinary ketone bodies are positive. Fungal elements are found on a KOH mount of the discharge. Which of the following statements best describes the organism responsible for this patient’s condition?

Histopathological examination shows non-septate branching hyphae

It appears as a narrow-based budding yeast with a thick capsule

It has budding and filamentous forms

Histopathological examination shows acute angle branching hyphae

A 16-year-old girl presents to her physician with itching, soreness, and irritation in the vulvar region. She reports that these episodes have occurred 6–7 times a year since the age of 5. She used to treat these symptoms with topical ketoconazole cream, but this time it failed to help. She also has had several episodes of oral candidiasis in the past. She is not sexually active and does not take any medication. Her vital signs are as follows: the blood pressure is 115/80 mm Hg, the heart rate is 78/min, the respiratory rate is 15/min, and the temperature is 35.5°C (97.7°F). Examination shows vulvovaginal erythema with cottage cheese-like plaques and an intact hymen. Wet mount microscopy is positive for yeast. Along with a swab culture, the physician orders a dihydrorhodamine test and myeloperoxidase staining for a suspected primary immunodeficiency. The dihydrorhodamine test is positive, and the myeloperoxidase staining reveals diminished staining. Which of the following best describes this patient's condition?

The patient is likely to have another immune impairment besides the one for which she was tested.

The patient should receive prophylactic courses of wide spectrum antibiotics to prevent infections.

The patient’s phagocytes are unable to generate an oxidative burst to kill intracellular bacteria.

The patient is susceptible to all mycotic infections.

The patient’s phagocytes can only perform extracellular killing.

Aspergillus species for USMLE Step 3: High-Yield Microbiology Lessons

Aspergillus ID - Fungus Among Us

Morphology: Mold with septate hyphae.
- Acute-angle (45°) V-shaped branching.
- Not dimorphic (exists only as a mold).
Reproduction: Asexual conidiospores on a conidiophore structure.

⭐ Aspergillus is often confused with Mucor, but Mucor has non-septate hyphae and branches at 90° angles.

Pathogenesis - A Spore Story

Inhalation: Spores (conidia) are ubiquitous in the environment (air, soil, decaying matter) and are inhaled into the lungs.
Virulence Factors:
- Catalase: Neutralizes oxidative burst from phagocytes, allowing survival.
- Aflatoxin: Produced by A. flavus, it is a potent hepatotoxin and carcinogen linked to hepatocellular carcinoma.

⭐ In immunocompromised patients, hyphae can invade blood vessels (angioinvasion), leading to thrombosis, infarction, and distant dissemination.

Clinical Syndromes - The Fungal Frontier

Aspergillus causes a spectrum of disease based on host immunity, from allergic reactions to direct invasion.

Syndrome	Patient Profile	Pathophysiology & Key Features
Allergic Bronchopulmonary Aspergillosis (ABPA)	Asthma, Cystic Fibrosis	Type I/IV Hypersensitivity. Wheezing, migratory pulmonary infiltrates, central bronchiectasis. ↑IgE, ↑eosinophils.
Aspergilloma	Pre-existing lung cavity (TB, sarcoidosis)	Fungus ball (mycetoma) in cavity. Often asymptomatic; may cause hemoptysis. X-ray shows a mass that shifts with position.
Invasive Aspergillosis	Immunocompromised (neutropenia, steroids)	Vascular invasion & tissue necrosis. Fever, pleuritic pain, hemoptysis. Triad: fever, pleuritic pain, hemoptysis.

⭐ Angioinvasion is the hallmark of invasive aspergillosis. Look for the halo sign (ground-glass opacity surrounding a nodule) on CT as an early sign of hemorrhage.

Diagnosis - Spotting the Spores

Microscopy: Biopsy reveals septate hyphae with acute (45°) angle branching.
- Stains: Gomori methenamine-silver (GMS) is classic.
Culture: Grows on Sabouraud agar; forms characteristic conidiophores.
Serology: Detects cell wall components.
- Galactomannan assay (antigen)
- Beta-D-glucan assay (non-specific)
Imaging: Chest X-ray or CT may show:
- Aspergilloma: A "fungus ball" in a pre-existing lung cavity.
- Allergic Bronchopulmonary Aspergillosis (ABPA): Eosinophilia, ↑IgE.
- Invasive Aspergillosis: Halo sign or air-crescent sign.

⭐ In tissue, Aspergillus is identified by its septate hyphae that branch at a 45° acute angle, distinguishing it from Mucor species (non-septate, 90° branching).

Aspergillus species: hyphae, conidiophore, and colony

Treatment - Fighting the Fungus

Treatment strategy is tailored to the specific clinical syndrome. For ABPA, the goal is to control the underlying hypersensitivity reaction, whereas invasive disease requires aggressive systemic antifungal therapy to eradicate the pathogen.

Condition	Primary Treatment	Key Considerations
Allergic Bronchopulmonary Aspergillosis (ABPA)	Systemic Corticosteroids	Long-term oral prednisone is the mainstay. Itraconazole may be added as a steroid-sparing agent.
Aspergilloma (Fungus Ball)	Surgical Resection	Reserved for patients with significant symptoms like massive hemoptysis. Observation is appropriate for asymptomatic cases.
Invasive Aspergillosis	Voriconazole	Drug of choice. Alternatives include isavuconazole or lipid formulations of Amphotericin B.

High-Yield Points - ⚡ Biggest Takeaways

Aspergillus fumigatus is a mold with septate hyphae that branch at acute, 45° angles.

Allergic Bronchopulmonary Aspergillosis (ABPA) is a hypersensitivity reaction in asthma/CF patients, with high IgE.

Aspergillomas are "fungus balls" that colonize pre-existing lung cavities (e.g., from TB).

Invasive aspergillosis affects the immunocompromised (especially neutropenic), showing angioinvasion and halo sign on CT.

Produces aflatoxins, which are highly hepatocarcinogenic.

Key diagnostic clue: positive galactomannan antigen test.

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