Nocardia asteroides

1. GC 102 (Fever after chemotherapy / infections in immunocompromised hosts): Nocardia is listed among opportunistic infections in neutropenic and cell-mediated immunodeficient patients. The lecture emphasises that pulmonary Nocardia is a key differential for abnormal lung shadow in immunocompromised patients alongside Aspergillus, PJP, CMV, and TB ^[1].

2. GC 105 (Medically important microbes — what every doctor should know): Nocardia is highlighted as a weakly acid-fast aerobic actinomycete — the acid-fast property and aerobic nature are the two testable discriminators from Actinomyces ^[2].

3. GC 052 (Fever and purulent sputum): Nocardia should be considered when a patient with immunosuppression presents with subacute purulent sputum and cavitary CXR changes that do not respond to standard empirical antibiotics ^[1].

4. TMP-SMX dual coverage: In immunocompromised patients already on TMP-SMX prophylaxis for PJP, they are simultaneously protected against Nocardia. If prophylaxis is stopped or the patient is non-compliant, Nocardia risk rises — this is a frequently tested concept in post-transplant infection scenarios ^[1][2].

5. Jerry's immunodeficiencies (Senior notes): Chronic granulomatous disease (CGD) patients are susceptible to catalase-positive organisms; Nocardia is catalase-positive and can cause disease in CGD, though Staphylococcus and Aspergillus are more classic ^[4].

6. Modified acid-fast staining: The distinction between standard ZN (20% H₂SO₄ — for Mycobacteria) and modified ZN (1% H₂SO₄ — for Nocardia) is a classic MCQ discriminator. Nocardia loses staining with strong acid but retains it with weak acid ^[3].

• Nocardia vs Actinomyces — the single most common exam trap:

• Nocardia vs Rhodococcus equi: Both are aerobic, partially acid-fast, and cause pulmonary disease in immunocompromised. Rhodococcus = coccobacillus (not filamentous), classically in HIV with horse/farm exposure.

• Nocardia vs Aspergillus: Both cause cavitary lung disease and can disseminate to brain in immunocompromised. Nocardia = bacterium (responds to TMP-SMX); Aspergillus = fungus (responds to voriconazole/amphotericin). Gram stain and modified acid-fast stain differentiate.

• "Branching filamentous rods" in sputum of an immunocompromised patient = Nocardia until proven otherwise.

• Brain abscess differential in immunocompromised: Nocardia (weakly acid-fast branching rods), Toxoplasma (ring-enhancing, HIV), Aspergillus (fungal hyphae), TB (caseating granuloma).

2021 Fourth Summative SAQ Q12 ^[5]:

"A 40-year-old man had bone marrow transplant for chronic myeloid leukaemia. ... (b) On day 28 after neutrophil engraftment, he developed fever, impaired liver function test, persistent diarrhoea and chest X-ray infiltrates. The patient admitted that he did not take the antimicrobial prophylaxis which was prescribed. List 四 organisms which may account for his symptoms."

• Correct answer / markscheme: The question asks for 4 organisms in a post-BMT patient who stopped prophylaxis (likely TMP-SMX). Expected answers include CMV (reactivation causing colitis, hepatitis, pneumonitis), PJP (Pneumocystis jirovecii — CXR infiltrates, no prophylaxis), Nocardia (lung infiltrates, not taking TMP-SMX prophylaxis), and Aspergillus (pulmonary infiltrates post-engraftment).
• Discriminator: The vignette's key clue is "did not take antimicrobial prophylaxis" — TMP-SMX prophylaxis covers both PJP and Nocardia. Non-compliance unmasks risk for both. This is a classic exam hook tying Nocardia to the immunocompromised host and TMP-SMX prophylaxis ^[1][5].

No other directly Nocardia-specific past paper questions were identified in the indexed past papers.