Tuberculosis (TB) is one of those Step 1 topics that feels deceptively simple (“acid-fast bug, chronic cough”) until you realize the exam writers are really testing immunology + pathology + radiology patterns + pharmacology—especially the distinction between primary TB and reactivation (secondary) TB. This post is your high-yield, concept-first map for mastering both.
The organism (what Step 1 actually cares about)
Mycobacterium tuberculosis
- Acid-fast bacillus (AFB) due to mycolic acids in the cell wall
- Obligate aerobe → prefers high O₂ areas (classic: lung apices)
- Slow-growing → prolonged treatment and delayed culture growth
- Cord factor (trehalose dimycolate) and sulfatides help inhibit phagolysosome fusion and promote survival in macrophages
First Aid cross-reference: Microbiology → Mycobacteria (M. tuberculosis), Pulmonary → TB; Pharmacology → TB drugs (RIPE)
Big picture: primary vs reactivation TB
High-yield comparison table
| Feature | Primary TB | Reactivation (Secondary) TB |
|---|---|---|
| Typical host | Often children or first exposure; may be asymptomatic | Prior infection with waning immunity |
| Location | Lower part of upper lobe or upper part of lower lobe (often mid-lung) | Apical/posterior upper lobes (high O₂) |
| Key lesion | Ghon focus; Ghon complex (focus + hilar nodes) | Cavitary lesions are classic |
| LN involvement | Hilar lymphadenopathy common | Less prominent |
| Path | Initial infection → immune response weeks later | Dormant bacilli in granulomas “wake up” |
| Transmission | Can transmit if active pulmonary disease develops | Often highly infectious (coughing AFB) |
| Symptoms | Often mild; may be flu-like | Chronic cough, hemoptysis, night sweats, weight loss |
| Complications | Usually heals with fibrosis/calcification | Hemoptysis, pleural effusions, miliary spread |
Pathophysiology (this is the core Step 1 mechanism)
1) Entry and early infection
TB is inhaled → reaches alveoli → taken up by alveolar macrophages. The bacteria can survive because they prevent phagolysosome fusion.
2) The “turning point”: Type IV hypersensitivity (delayed)
After ~2–4 weeks, the immune system mounts a Th1 response:
- Macrophages release IL-12 → drives Th1 differentiation
- Th1 cells release IFN-γ → activates macrophages to kill intracellular organisms
- Activated macrophages and T cells form granulomas
Granulomas in TB
- Caseating granulomas: central “cheesy” necrosis
- Langhans giant cells: multinucleated macrophages with nuclei arranged at the periphery (“horseshoe”)
Key Step 1 concept:
Symptoms and tissue damage in TB are largely driven by the host immune response, not just direct bacterial toxins.
First Aid cross-reference: Immunology → Th1/IFN-γ/macrophage activation; Pathology → granulomatous inflammation
Primary TB (what it is and what to recognize)
Definition
Primary TB = the initial infection in someone not previously exposed (or not effectively immunized/immune).
Classic lesions
- Ghon focus: subpleural lesion, typically in mid-lung regions
- Ghon complex: Ghon focus + ipsilateral hilar lymph node involvement
Most primary infections are contained:
- Lesions may undergo fibrosis and calcification (can be seen on imaging)
- Latency can develop with dormant organisms inside granulomas
Clinical presentation
Often asymptomatic, or mild nonspecific symptoms:
- Low-grade fever, malaise
- Mild cough
- In kids: hilar lymphadenopathy can be a clue
Reactivation (secondary) TB (the board-style “classic TB”)
Definition
Reactivation TB = dormant bacilli resume replication when host immunity declines.
Why apices?
TB is an obligate aerobe, so it thrives in high oxygen tension → apical/posterior upper lobes.
Risk factors for reactivation (very high-yield)
Think: anything that weakens cell-mediated immunity.
- HIV/AIDS (especially low CD4)
- TNF-α inhibitors (e.g., infliximab, adalimumab)
- TNF-α is critical for maintaining granulomas
- Chronic corticosteroid use
- Diabetes mellitus
- Malnutrition
- Alcohol use disorder
- Silicosis
- Chronic kidney disease
- Post-transplant immunosuppression
First Aid cross-reference: Immunology/Pharm → TNF-α inhibitors; Micro → reactivation in immunosuppression
Classic symptoms (Step 1 loves these)
- Chronic cough
- Hemoptysis
- Fever
- Night sweats
- Weight loss
- ± pleuritic chest pain
Imaging
- Apical cavitary lesions are the hallmark
- May see fibrosis and volume loss in upper lobes
Disseminated TB and key complications (know these patterns)
Miliary TB
Hematogenous spread → tiny lesions throughout organs (“millet seeds”)
- Can involve liver, spleen, bone marrow, and lungs
- Often in immunocompromised patients
- Systemic symptoms, possible sepsis-like picture
Pott disease (TB osteomyelitis)
- TB of the vertebrae → vertebral collapse, back pain
- Can cause spinal deformity and neurologic compromise
TB meningitis
- Subacute meningitis; cranial nerve palsies possible
- CSF tends to have:
- ↑ protein, ↓ glucose
- Lymphocytic predominance (classically)
Scrofula
- TB cervical lymphadenitis (classically painless neck swelling)
Diagnosis: tests you must be able to interpret
1) Screening for infection: TST vs IGRA
Tuberculin skin test (PPD)
- Intradermal purified protein derivative
- Read at 48–72 hours
- Positive = type IV hypersensitivity (Th1-mediated)
False positives
- BCG vaccination
- Infection with non-tuberculous mycobacteria
False negatives
- Early infection (before immune response develops)
- Severe immunosuppression (e.g., advanced HIV)
- Very young/very old; severe illness
Interferon-gamma release assay (IGRA)
- Measures IFN-γ release by T cells exposed to TB antigens
- More specific than PPD in BCG-vaccinated patients
- Doesn’t distinguish active vs latent infection
High-yield: If they mention BCG and want the best test for latent TB infection → IGRA is typically favored.
2) Confirming active pulmonary TB
Sputum testing
- Acid-fast smear (Ziehl–Neelsen or auramine-rhodamine)
- Culture (slow; but gold standard for viability and drug susceptibility)
- NAAT/PCR (rapid detection; can identify resistance markers depending on assay)
Chest imaging
- Reactivation: apical cavitation
- Primary: hilar adenopathy, mid-lung lesions
Important Step-style nuance:
A positive PPD/IGRA indicates infection (often latent), not necessarily active disease. Active disease needs symptoms + imaging and microbiologic evidence.
Treatment (RIPE, resistance, and toxicities)
Active TB (initial empiric therapy)
Standard initial regimen is RIPE:
- Rifampin
- Isoniazid
- Pyrazinamide
- Ethambutol
After initial phase, therapy is typically narrowed based on sensitivities (often continuing INH + RIF).
Why multiple drugs?
Prevents selection of resistant mutants and treats different bacterial populations (intracellular/extracellular, slow-growing, etc.).
TB drug toxicities (these are pure Step 1 points)
| Drug | Mechanism (HY) | Major adverse effects | Key “fix” / note |
|---|---|---|---|
| Rifampin | Inhibits DNA-dependent RNA polymerase | Hepatotoxicity, potent CYP450 inducer, orange body fluids | Drug interactions (OCPs, warfarin, ART) |
| Isoniazid (INH) | Inhibits mycolic acid synthesis | Hepatotoxicity, peripheral neuropathy, drug-induced lupus | Give vitamin B6 (pyridoxine) to prevent neuropathy |
| Pyrazinamide | Mechanism classically: disrupts membrane/energy; active in acidic pH | Hepatotoxicity, hyperuricemia (gout) | Think “PZA → Pain in joints” |
| Ethambutol | Inhibits arabinosyltransferase → ↓ cell wall | Optic neuritis (↓ visual acuity, red-green color blindness) | “E” for Eye toxicity |
First Aid cross-reference: Pharm → antimycobacterial drugs and toxicities
Latent TB treatment (commonly tested conceptually)
Latent TB = positive PPD/IGRA with no clinical/radiographic evidence of active disease.
- Common regimen: isoniazid (often with pyridoxine) or rifamycin-based regimens depending on scenario/guidelines
- Must rule out active TB first (don’t treat “latent” TB with monotherapy if active disease is possible)
Step 1 framing: latent TB = dormant organisms contained by immunity; treat to prevent reactivation, especially in high-risk patients.
High-yield associations and “exam-writer tells”
When they’re hinting reactivation TB
- Upper lobe/apical findings
- Cavitary lesions
- Night sweats, weight loss, hemoptysis
- History of immunosuppression (HIV, TNF-α inhibitors, steroids)
- Social risk factors: incarceration, homelessness, close-contact settings
When they’re hinting primary TB
- Child with hilar lymphadenopathy
- Ghon complex on imaging
- Recent exposure, mild symptoms
When they’re hinting miliary TB
- Diffuse tiny nodules on chest imaging
- Multi-organ involvement symptoms
- Significant immunosuppression
TNF-α inhibitors = granuloma breakdown
If a stem says “on infliximab/adalimumab” and then chronic cough/fever → think reactivation TB. This is one of the cleanest immunology-to-clinical bridges on Step exams.
Rapid “Primary vs Reactivation” memory anchors
- Primary: Ghon (think “G for Goes to hilar nodes” → Ghon complex)
- Reactivation: Apex + cavitation + systemic “B symptoms”
- Immunity failure drives reactivation: HIV, TNF-α inhibitors, steroids
Mini practice prompts (to self-test)
- Patient on infliximab develops chronic cough + night sweats + apical cavitation → what’s the diagnosis and why apex?
- Child with mid-lung lesion + hilar adenopathy after exposure → what complex is this?
- Which TB drug causes optic neuritis? Which causes neuropathy and how do you prevent it?
- Why can BCG vaccination cause a false-positive PPD but not IGRA?
Take-home summary (what you should recall in 20 seconds)
- TB causes caseating granulomas via Th1 → IFN-γ macrophage activation (type IV hypersensitivity).
- Primary TB: Ghon focus/complex, often mid-lung + hilar nodes; can become latent.
- Reactivation TB: apical cavitary disease with B symptoms; triggered by weakened cell-mediated immunity (HIV, TNF-α inhibitors, steroids).
- Diagnose active disease with sputum (AFB/NAAT/culture) + imaging; PPD/IGRA show infection, not necessarily active disease.
- Treat active TB with RIPE; know toxicities: RIF orange/CYP, INH neuropathy (B6), PZA hyperuricemia, EMB optic neuritis.