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 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

Mucormycoses for USMLE Step 1: High-Yield Microbiology Lessons

Microbiology & Risk Factors - The Moldy Menace

Causative Agents: Mucor, Rhizopus, Lichtheimia (formerly Absidia) species.
Microscopic Morphology:
- Broad, ribbon-like, non-septate (pauciseptate) hyphae.
- Branching occurs at wide, often right angles (~90°).
Major Risk Factors:
- Diabetic Ketoacidosis (DKA): Hyperglycemia & acidosis create a favorable environment.
- Immunosuppression: Especially neutropenia and high-dose corticosteroid therapy.
- Iron Overload: Deferoxamine use paradoxically ↑ risk by acting as a fungal siderophore.

Mucormycosis histology: H&E and GMS stains

⭐ In DKA, acidosis causes dissociation of iron from binding proteins, and hyperglycemia enhances fungal growth, creating a perfect storm for Mucor invasion.

Pathophysiology - Angioinvasive Attack

Host Factors: Thrives in high-glucose, acidic environments. Key risks: Diabetic Ketoacidosis (DKA), neutropenia, iron overload (deferoxamine use).
Angioinvasion: Hyphae have a predilection for invading blood vessels, leading to thrombosis, infarction, and subsequent tissue necrosis.

⭐ The fungus possesses ketone reductase systems, allowing it to thrive in the high glucose, acidic conditions of DKA, a classic USMLE association.

Mucormycosis: Broad, non-septate hyphae invading tissue

Rhinocerebral Mucormycosis - A Sinister Spread

Primarily affects immunocompromised hosts, especially with Diabetic Ketoacidosis (DKA). Fungi (Rhizopus, Mucor) invade blood vessels, causing thrombosis and tissue necrosis.

Presentation: Acute sinusitis with fever, facial pain, headache.
Hallmark Sign: Black necrotic eschar on the nasal mucosa or hard palate.
Progression: Rapidly spreads through the cribriform plate to the brain.
- Leads to cavernous sinus thrombosis, proptosis, and cranial nerve palsies.

⭐ In DKA, high glucose and acidic pH impair neutrophil function, while excess free iron (released from transferrin) acts as a critical growth factor for the fungi.

Mucorales: Environment to Pathogen and Infection Routes

Diagnosis & Management - ID and Intervene

Diagnosis:
- Histopathology: Key for rapid diagnosis. Biopsy shows broad, non-septate hyphae with wide-angle (90°) branching.
- Culture: Confirms organism, but slow and may be falsely negative.
- Imaging: CT/MRI to assess the extent of angioinvasion and tissue necrosis.

Mucormycosis Histopathology: H&E and GMS Stains

Management:
- Emergent Surgical Debridement: Aggressive removal of all necrotic tissue is critical.
- Antifungal Therapy: High-dose IV Liposomal Amphotericin B is first-line. Step-down to oral posaconazole or isavuconazole.
- Control Risk Factors: Reverse DKA, manage neutropenia.

⭐ The presence of a black necrotic eschar in the nasal cavity or on the hard palate of a diabetic (especially DKA) or immunocompromised patient is highly suggestive of rhinocerebral mucormycosis.

Primarily affects immunocompromised hosts, especially those with diabetic ketoacidosis (DKA) and neutropenia.

Caused by fungi like Rhizopus, Mucor, and Lichtheimia species.

Histopathology reveals broad, non-septate hyphae with right-angle (90°) branching.

Classic presentation is rhino-orbital-cerebral infection, often with a black necrotic eschar on the palate or nasal turbinates.

Diagnosis requires biopsy for histology and culture.

Treatment is emergent: aggressive surgical debridement and systemic amphotericin B.

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Mucormycoses