Chronic granulomatous disease (CGD) is one of those “classic” immunodeficiencies that shows up everywhere—question stems, answer choices, and real-life patients with recurrent infections that never quite fit the usual pattern. The trick isn’t just recognizing CGD; it’s knowing why it’s CGD and why everything else isn’t. That’s where you start turning Q-bank misses into automatic points.
Tag: Immunology > Immunodeficiencies
The Clinical Vignette (Q-bank style)
A 6-year-old boy has had multiple infections since early childhood, including recurrent pneumonia and skin abscesses. Cultures have grown Serratia marcescens and Staphylococcus aureus. His mother reports a prior episode of osteomyelitis after minor trauma. On exam, he has cervical lymphadenopathy and healed scars from prior drained abscesses. Neutrophil oxidative burst testing is abnormal.
Question: The underlying defect most likely involves which of the following?
A. Defective NADPH oxidase in phagocytes
B. Decreased adenosine deaminase activity
C. Absence of CD18 integrins on neutrophils
D. Defective IFN-γ receptor signaling in macrophages
E. Defective Bruton's tyrosine kinase (BTK)
The Correct Answer: A. Defective NADPH oxidase in phagocytes (CGD)
What’s happening physiologically?
CGD is due to impaired respiratory burst in phagocytes—most commonly X-linked mutation in NADPH oxidase. Without NADPH oxidase, phagocytes can’t generate reactive oxygen species (ROS) effectively, so they struggle to kill certain organisms after phagocytosis.
The key cascade (Step 1 favorite)
- NADPH oxidase helps generate superoxide ()
- → → other ROS (e.g., bleach in neutrophil phagolysosomes)
- ROS support intracellular killing
If the bug produces its own , the host can sometimes “borrow” it to generate downstream ROS. Catalase-positive organisms break down their own , so CGD patients lose that workaround → recurrent infections.
High-yield associations
CGD → Catalase-positive organisms + granulomas
- Catalase-positive:
- S. aureus
- Serratia
- Burkholderia cepacia
- Nocardia
- Aspergillus (major cause of death—think pneumonia)
- Granuloma formation: chronic inflammation from persistent organisms
- Deep-seated abscesses, pneumonia, osteomyelitis (especially S. aureus, Serratia)
Diagnostic tests (know the direction of change)
| Test | CGD Result | What it means |
|---|---|---|
| Dihydrorhodamine (DHR) flow cytometry | Decreased fluorescence | ↓ oxidative burst |
| Nitroblue tetrazolium (NBT) test | Fails to turn blue (remains colorless) | no superoxide to reduce dye |
Treatment / prevention (very testable)
- TMP-SMX prophylaxis
- Itraconazole (or other antifungal prophylaxis depending on risk)
- Interferon-γ can reduce frequency of severe infections
- Curative option in select cases: hematopoietic stem cell transplant
Why Each Distractor Is Wrong (and what it really describes)
B. Decreased adenosine deaminase activity → SCID (ADA deficiency)
Why it’s tempting: “Recurrent infections in a child” can scream SCID.
Why it’s wrong here: SCID typically presents earlier and more broadly with severe viral, bacterial, and fungal infections, plus failure to thrive.
High-yield SCID clues
- ADA deficiency → toxic purine metabolites → lymphocyte death
- Very low T cells (and often B cells/NK depending on subtype)
- Absent thymic shadow
- Chronic diarrhea (e.g., opportunistic infections), thrush, severe viral infections
Buzzphrase: “No thymus, no T cells, everything infections.”
C. Absence of CD18 integrins on neutrophils → Leukocyte Adhesion Deficiency (LAD type 1)
Why it’s tempting: Neutrophil-related disorder causing bacterial infections.
Why it’s wrong here: LAD is primarily a trafficking problem—not a killing problem. You don’t see pus because neutrophils can’t extravasate.
High-yield LAD clues
- Delayed separation of umbilical cord (>30 days)
- No pus, impaired wound healing
- Recurrent bacterial skin/mucosal infections
- Leukocytosis with neutrophilia (neutrophils stuck in blood)
Step hint: If you see “no pus + delayed cord separation,” think LAD, not CGD.
D. Defective IFN-γ receptor signaling in macrophages → Mendelian susceptibility to mycobacterial disease / Disseminated atypical mycobacteria
Why it’s tempting: Granulomas make people think “mycobacteria,” and IFN-γ is key for macrophage activation.
Why it’s wrong here: The stem emphasizes catalase-positive pyogenic organisms like Serratia and S. aureus and an abnormal oxidative burst test—classic CGD. IFN-γ/IL-12 axis defects skew toward disseminated mycobacterial and Salmonella infections.
High-yield IL-12/IFN-γ axis
- Macrophages make IL-12 → stimulates Th1 cells
- Th1 cells make IFN-γ → activates macrophages
- Defects → poor intracellular control of mycobacteria (including BCG vaccine complications) and Salmonella
Clue: “Disseminated atypical mycobacteria after BCG” → IL-12/IFN-γ pathway.
E. Defective Bruton's tyrosine kinase (BTK) → X-linked agammaglobulinemia (Bruton)
Why it’s tempting: Male child + recurrent infections.
Why it’s wrong here: Bruton is a B-cell maturation problem with absent immunoglobulins and absent lymphoid tissue development—especially after maternal IgG wanes (~6 months). The hallmark organisms skew toward encapsulated bacteria and enteroviruses.
High-yield Bruton clues
- Presents after 6 months
- Low/absent B cells (CD19+) and all immunoglobulins
- Absent germinal centers, small/absent tonsils and lymph nodes
- Infections with encapsulated bacteria (S. pneumoniae, H. influenzae) + Giardia
- Enterovirus risk (polio/echo/coxsackie)
Step move: Bruton = “No B cells, no antibodies, no germinal centers.”
The “CGD Pattern” You Should Be Able to Spot in 10 Seconds
Stem features that strongly point to CGD
- Recurrent catalase-positive infections (especially Serratia, Aspergillus, S. aureus, Nocardia)
- Deep abscesses, pneumonia, osteomyelitis
- Mention of granulomas
- Abnormal DHR or NBT test
- Often male (X-linked most common)
One-liner you can use on test day
CGD = NADPH oxidase defect → ↓ respiratory burst → catalase-positive infections + granulomas; DHR ↓ fluorescence, NBT negative.
Rapid-Fire High-Yield Table: Differentiate the Big Five
| Disorder | Core defect | Key clue | Classic organisms / findings |
|---|---|---|---|
| CGD | NADPH oxidase | Catalase+ infections, granulomas | S. aureus, Serratia, Nocardia, Aspergillus |
| SCID (ADA) | Purine metabolism → lymphotoxic | Early severe infections, no thymic shadow | Candida, PJP, viral infections |
| LAD-1 | CD18 (integrin) | Delayed cord separation, no pus, neutrophilia | Skin/mucosal bacterial infections |
| IFN-γ/IL-12 axis | Macrophage activation impaired | Disseminated mycobacteria, BCG complication | Atypical mycobacteria, Salmonella |
| Bruton (XLA) | BTK → no mature B cells | After 6 months, absent tonsils | Encapsulated bacteria, Giardia, enteroviruses |
Takeaway: How to “Win” These Questions
- Anchor the diagnosis with 1–2 stem specifics (here: Serratia + abnormal oxidative burst).
- Name the mechanism (NADPH oxidase → no respiratory burst).
- Use distractors as pattern recognition practice (LAD = no pus; SCID = very early + thrush; Bruton = absent tonsils; IFN-γ = mycobacteria).
That’s the difference between “I’ve heard of CGD” and “I can’t miss CGD.”