A 36-year-old woman comes to the emergency department because of an itchy lesion on her skin. The rash developed shortly after she took an antibiotic for a urinary tract infection. Her temperature is 37.5°C (99.3°F), pulse is 99/min, and blood pressure is 100/66 mm Hg. Physical examination shows swelling of the face and raised, erythematous plaques on her trunk and extremities. Which of the following is the most likely cause of this patient's current condition?

Secretion of cytokines by T cells

Activation of complement cascade

A 15-year-old boy is admitted to the emergency department with neck stiffness, maculopapular rash, fever, and a persistent headache. A blood culture shows encapsulated gram-negative diplococci. He has had this same infection before. Which of the following proteins is likely to be deficient in this patient?

CD55 (decay accelerating factor)

A 38-year-old woman comes to the physician for a follow-up examination. Two years ago, she was diagnosed with multiple sclerosis. Three weeks ago, she was admitted and treated for right lower leg weakness with high-dose methylprednisone for 5 days. She has had 4 exacerbations over the past 6 months. Current medications include interferon beta and a multivitamin. Her temperature is 37°C (98.6°F), pulse is 90/min, and blood pressure is 116/74 mm Hg. Examination shows pallor of the right optic disk. Neurologic examination shows no focal findings. She is anxious about the number of exacerbations and repeated hospitalizations. She is counseled about the second-line treatment options available to her. She consents to treatment with natalizumab. However, she has read online about its adverse effects and is concerned. This patient is at increased risk for which of the following complications?

Progressive multifocal leukoencephalopathy

Syndrome of inappropriate antidiuretic hormone

A 28-year-old woman has a follow-up visit with her physician. She was diagnosed with allergic rhinitis and bronchial asthma at 11 years of age. Her regular controller medications include daily high-dose inhaled corticosteroids and montelukast, but she still needs to use a rescue inhaler 3–4 times a week following exercise. She also becomes breathless with moderate exertion. After a thorough evaluation, the physician explains that her medication dosages need to be increased. She declines taking oral corticosteroids daily due to concerns about side effects. The physician prescribes omalizumab, which is administered subcutaneously every 3 weeks. Which of the following best explains the mechanism of action of the new medication that has been added to the controller medications?

Prevention of binding of IgE antibodies to mast cell receptors

Inhibition of synthesis of interleukin-4 (IL-4)

Inhibition of synthesis of IgE antibodies

Selective binding to interleukin-3 (IL-3) and inhibition of its actions

Prevention of binding of interleukin-5 (IL-5) to its receptors

You are treating a neonate with meningitis using ampicillin and a second antibiotic, X, that is known to cause ototoxicity. What is the mechanism of antibiotic X?

It binds the 30S ribosomal subunit and inhibits formation of the initiation complex

It binds the 50S ribosomal subunit and inhibits formation of the initiation complex

It binds the 30S ribosomal subunit and reversibly inhibits translocation

It binds the 50S ribosomal subunit and inhibits peptidyltransferase

It binds the 50S ribosomal subunit and reversibly inhibits translocation

A 28-year-old female presents to her primary care doctor complaining of new onset blurry vision. She first noticed her vision getting blurry toward the end of the day several days ago. Since then, she reports that her vision has been fine when she wakes up but gets worse throughout the day. She has also noticed that her eyelids have started to droop before she goes to bed. On exam, she has bilateral ptosis that is worse on the right. Administering edrophonium to this patient leads to an immediate improvement in her symptoms. Which of the following is most likely true about this patient’s condition?

It is associated with a benign proliferation of epithelial cells of the thymus

It is caused by a type III hypersensitivity reaction

It is associated with a neoplasm of lung neuroendocrine cells

It is caused by antibodies directed against presynaptic P/Q calcium channels

An increasing response will be seen on repeated nerve stimulation

Complement inhibitors for USMLE Step 3: High-Yield Pharmacology Lessons

Complement Cascade - The Body's Bouncer

A system of plasma proteins that "complements" the work of antibodies.
Three main pathways: Classical (IgG/IgM), Lectin (mannose), and Alternative (pathogen surfaces).

Complement pathways and inhibitor drug targets

Key Effector Functions:
- Opsonization: C3b acts as an "eat me" signal for phagocytes.
- Inflammation: C3a and C5a are anaphylatoxins, recruiting neutrophils.
- Cytolysis: The Membrane Attack Complex (MAC) punches holes in pathogen membranes.

⭐ Patients with deficiencies in the terminal complement components (C5-C9) are highly susceptible to recurrent infections with encapsulated bacteria, especially Neisseria species.

Mechanism of Action - Putting a Lid on It

Complement inhibitors block key proteins in the complement cascade, preventing downstream inflammation and cell lysis.
C5 Inhibitors (Eculizumab, Ravulizumab):
- Bind to complement protein C5, preventing its cleavage to C5a and C5b.
- This halts the formation of the Membrane Attack Complex (MAC), C5b-9, which creates pores in cell membranes.
C3 Inhibitor (Pegcetacoplan):
- Acts earlier, blocking C3 cleavage, thereby inhibiting all downstream effector functions.

Complement cascade with C3 and C5 activation

⭐ By preventing MAC formation, C5 inhibitors significantly increase susceptibility to encapsulated bacteria, especially Neisseria meningitidis. Vaccination is mandatory before starting treatment.

The Inhibitors - Meet the Suppressors

C5 Inhibitors (Terminal Pathway Blockade)
- -zumab drugs: Eculizumab, Ravulizumab
- Mechanism: Monoclonal antibodies that bind C5, preventing its cleavage to C5a (pro-inflammatory anaphylatoxin) & C5b (initiates MAC). This halts MAC-mediated intravascular hemolysis.
- Uses: Paroxysmal Nocturnal Hemoglobinuria (PNH), Atypical Hemolytic Uremic Syndrome (aHUS), Myasthenia Gravis (MG).
- Ravulizumab: Longer-acting, less frequent dosing.
- ⚠️ Adverse Effect: ↑ risk of encapsulated bacterial infections.
  
  ⭐ High-Yield: Patients on C5 inhibitors require vaccination against Neisseria meningitidis (serogroups A, C, W, Y, and B) due to impaired MAC-dependent bacterial clearance.
C3 Inhibitors (Central Blockade)
- Pegcetacoplan
- Mechanism: Binds C3 & C3b, controlling cleavage. Addresses both intravascular and extravascular hemolysis.
- Use: PNH.
C1-Esterase (C1-INH) Inhibitors
- Mechanism: Replaces deficient C1-INH in Hereditary Angioedema (HAE), preventing unregulated kallikrein activation and bradykinin production.
- Use: Hereditary Angioedema (HAE) attacks.

Clinical Uses & Risks - The Trade-Offs

Primary Uses
- Paroxysmal Nocturnal Hemoglobinuria (PNH)
- Atypical Hemolytic Uremic Syndrome (aHUS)
- Generalized Myasthenia Gravis (gMG) (anti-AChR Ab+)
- Neuromyelitis Optica Spectrum Disorder (NMOSD)
Major Risks & Mitigation
- ⚠️ BLACK BOX WARNING: Increased risk of life-threatening meningococcal infections (N. meningitidis).
- Requires vaccination at least 2 weeks prior to initiation.
- Headache, hypertension, and infusion-related reactions are common.

⭐ Eculizumab & Ravulizumab bind C5, preventing its cleavage to C5a and C5b. This halts MAC formation, protecting RBCs in PNH.

High‑Yield Points - ⚡ Biggest Takeaways

Eculizumab and Ravulizumab are monoclonal antibodies that bind C5, preventing the formation of the Membrane Attack Complex (MAC).

They are primarily used for Paroxysmal Nocturnal Hemoglobinuria (PNH) and atypical Hemolytic Uremic Syndrome (aHUS).

The most significant risk is life-threatening Neisseria meningitidis infections; vaccination is mandatory before starting treatment.

Sutimlimab is a C1s inhibitor used for hemolysis in cold agglutinin disease (CAD).

Pegcetacoplan, a C3 inhibitor, is also used for PNH.

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