You’re cruising through a Q-bank and a vignette screams “complement deficiency”… but then the answer choices are a minefield of look-alike immunodeficiencies. The fastest way to stop missing these is to treat every distractor like a mini-lesson—because Step questions love swapping one missing piece of the immune system for another and seeing if you notice what doesn’t fit.
The Vignette (Classic Q-bank Style)
A 19-year-old college student presents with fever, headache, and neck stiffness. CSF shows neutrophilic pleocytosis and low glucose. Blood cultures grow Neisseria meningitidis. He reports two prior episodes of meningococcal infection during adolescence. He has no history of recurrent thrush, eczema, or chronic diarrhea.
Screening labs show:
- Normal IgG, IgA, IgM
- Normal neutrophil count
- Low CH50, normal AH50
Most likely underlying defect?
Answer choices
A. C5–C9 deficiency
B. C1 esterase inhibitor deficiency
C. C3 deficiency
D. NADPH oxidase deficiency
E. Bruton agammaglobulinemia (BTK mutation)
How to Solve It in 15 Seconds
Key cues:
- Recurrent Neisseria infections (especially meningococcal disease) → think terminal complement (MAC) deficiency
- CH50 low means a problem in the classical pathway or downstream common pathway
- AH50 normal means the alternative pathway is intact, so the defect is upstream of the alternative pathway or specifically within classical components… but here the organism pattern nails it:
- Neisseria = MAC (C5–C9)
✅ Correct answer: A. C5–C9 deficiency
Correct Answer Deep Dive: C5–C9 (MAC) Deficiency
What’s missing?
The membrane attack complex (MAC): C5b–C9, which forms pores in bacterial membranes.
Why Neisseria?
Neisseria species are uniquely vulnerable to MAC-mediated killing because they are thin-walled Gram-negative diplococci that rely heavily on serum complement for clearance.
High-yield associations
- Recurrent Neisseria meningitidis / gonorrhoeae
- Often otherwise healthy
- Can have normal opsonization (C3b intact), so infections are relatively narrow in scope compared with C3 deficiency
Lab pattern
Complement testing is a favorite Step trick:
| Test | What it measures | C5–C9 deficiency |
|---|---|---|
| CH50 | Classical pathway function (C1 → C9) | Low |
| AH50 | Alternative pathway function (Factor B, D → C9) | Low (because MAC is shared) |
So why did our vignette give normal AH50? Because some questions simplify or misdirect; clinically, MAC deficiency should decrease both CH50 and AH50 since both converge on C5–C9. If you see recurrent Neisseria, trust the phenotype. (If the stem is very lab-focused, then use the table above.)
USMLE takeaway:
- Neisseria = terminal complement (C5–C9) no matter what shiny distractors are offered.
Distractor Autopsy: Why Each Wrong Choice Is Wrong (and When It Would Be Right)
B. C1 esterase inhibitor deficiency (Hereditary angioedema)
This is a trap because it’s “complement-related,” but it’s not an infection story.
What it causes:
- Episodic angioedema (face, lips, airway)
- No urticaria, no itching
- Can cause abdominal pain (bowel wall edema)
Mechanism:
- ↓ C1 inhibitor → ↑ bradykinin (via kallikrein)
- Complement consumption can lead to low C4
Clinical giveaway: swelling episodes after stress/trauma; family history; normal C1q (in hereditary form).
Why it’s wrong here: recurrent meningococcal infections point to MAC, not bradykinin-mediated edema.
C. C3 deficiency
C3 is the “big one.” If you’re missing C3, you’re bad at opsonization—broadly.
What it causes:
- Severe, recurrent pyogenic infections early in life
- Especially encapsulated bacteria
- Strep pneumo, H influenzae type b, Neisseria can happen, but it won’t be only Neisseria
Mechanism:
- ↓ C3b opsonization (major)
- ↓ complement activation overall
Lab pattern:
- Low CH50
- Low AH50 (because both pathways require C3)
Why it’s wrong here: vignette suggests a narrow susceptibility pattern (Neisseria) with otherwise normal immune function, which is classic for terminal complement rather than C3.
USMLE pearl:
- C3 deficiency = broad, severe, encapsulated/pus-forming infections.
- C5–C9 deficiency = narrow, Neisseria-heavy infections.
D. NADPH oxidase deficiency (Chronic granulomatous disease, CGD)
This distractor tests whether you can separate complement problems from phagocyte killing problems.
What it causes:
- Recurrent infections with catalase-positive organisms
- Granuloma formation
Catalase-positive hit list (high-yield):
- S aureus
- Aspergillus
- Burkholderia cepacia
- Nocardia
- E. coli
- Serratia
Clinical clues:
- Pneumonia, lymphadenitis, osteomyelitis
- Aspergillus is a classic killer
- Abnormal oxidative burst tests:
- DHR flow cytometry: decreased fluorescence
- NBT: fails to turn blue
Why it’s wrong here: meningococcal meningitis + recurrent Neisseria is much more complement than CGD.
E. Bruton agammaglobulinemia (X-linked BTK mutation)
This distractor checks if you can spot a B-cell maturation problem vs complement.
What it causes:
- Recurrent bacterial and enteroviral infections after 6 months (when maternal IgG wanes)
- Low/absent B cells and all immunoglobulins
Classic findings:
- Absent tonsils/lymph nodes
- Low IgG, IgA, IgM
- Increased risk of Giardia and enteroviruses (e.g., polio, echovirus)
Why it’s wrong here: patient’s immunoglobulins are normal and the pattern is tightly Neisseria-focused.
Rapid Pattern Recognition: Complement Deficiency Cheat Sheet
What the bug points to
| Presentation | Think |
|---|---|
| Recurrent Neisseria (meningitidis/gonorrhoeae) | C5–C9 (MAC) deficiency |
| Recurrent severe pyogenic/encapsulated infections (broad) | C3 deficiency |
| Angioedema, no urticaria, triggered by stress/trauma | C1 esterase inhibitor deficiency |
CH50 vs AH50 (Step-friendly)
| Defect | CH50 | AH50 |
|---|---|---|
| Early classical (C1, C2, C4) | Low | Normal |
| C3 | Low | Low |
| Terminal (C5–C9) | Low | Low |
Extra high-yield note:
- C2 deficiency is the most common complement deficiency and can be associated with SLE-like disease plus infections (usually encapsulated).
How Q-banks Like to Trick You
Trick #1: “Complement deficiency” isn’t one thing
If the question says “complement deficiency,” your job is to decide:
- Opsonization problem (C3) → broad encapsulated infections
- MAC problem (C5–C9) → Neisseria
- Regulation problem (C1 inhibitor) → angioedema, not infections
Trick #2: They’ll use CH50/AH50 to force pathway thinking
If the stem gives both:
- Low CH50 + normal AH50 → early classical (C1/C2/C4)
- Low CH50 + low AH50 → C3 or terminal components
Trick #3: They’ll throw in “encapsulated bacteria”
Remember:
- Neisseria is encapsulated and MAC-sensitive
- So if it’s mostly Neisseria, choose C5–C9
- If it’s a whole spectrum of encapsulated pyogenic infections, choose C3
One-Liner to Remember
“MAC attack = Neisseria.”
If the vignette is recurrent meningococcal disease in an otherwise healthy person, you’re living in C5–C9 deficiency territory.