Infectious Diseases and Immunology Flashcards

A 30-year-old man from sub-Saharan Africa presents with fever, weight loss, and a CD4 count of 150 cells/microL. Chest X-ray shows bilateral upper lobe cavitating lesions. Sputum is positive for acid-fast bacilli. What is the most appropriate initial treatment regimen?

Answer: Rifampicin, isoniazid, pyrazinamide, and ethambutol for 2 months then rifampicin and isoniazid for 4 months

Standard UK treatment for pulmonary TB is 2 months of RIPE (rifampicin, isoniazid, pyrazinamide, ethambutol) followed by 4 months of rifampicin and isoniazid (total 6 months). In HIV co-infection, antiretroviral therapy should be started within 2-8 weeks, with attention to drug interactions (rifampicin with protease inhibitors).

A healthcare worker sustains a needlestick injury from a patient known to be HIV positive with a detectable viral load. The injury is deep and from a hollow-bore needle. When should post-exposure prophylaxis (PEP) ideally be started?

Answer: Within 1 hour (as soon as possible)

HIV PEP should be started as soon as possible, ideally within 1 hour of exposure. The recommended regimen is a 28-day course of two NRTIs plus a third agent (usually raltegravir or dolutegravir). PEP is unlikely to be effective if started more than 72 hours after exposure.

A 5-year-old child presents with fever, petechial rash, and neck stiffness. CSF shows raised WCC (predominantly neutrophils), raised protein, low glucose, and Gram-negative diplococci. What is the most likely causative organism?

Answer: Neisseria meningitidis

Neisseria meningitidis (meningococcus) is a Gram-negative diplococcus and the most common cause of bacterial meningitis in children and young adults in the UK. The petechial/purpuric rash is characteristic of meningococcal disease. Close contacts require ciprofloxacin chemoprophylaxis.

A patient with rheumatoid arthritis on methotrexate and adalimumab develops reactivation of latent tuberculosis. Which component of the immune system has been most critically compromised?

Answer: TNF-alpha mediated macrophage activation and granuloma maintenance

TNF-alpha is essential for maintaining granulomas that contain latent Mycobacterium tuberculosis. Anti-TNF agents (adalimumab, infliximab, etanercept) disrupt granuloma integrity, allowing reactivation of latent TB. All patients must be screened for latent TB (with interferon-gamma release assay) before starting anti-TNF therapy.

A 70-year-old man develops profuse watery diarrhoea and abdominal cramping 5 days after completing a course of co-amoxiclav. Stool toxin assay is positive. What is the first-line treatment?

Answer: Oral vancomycin

NICE and PHE guidelines now recommend oral vancomycin (125 mg QDS for 10 days) as first-line treatment for all severities of Clostridioides difficile infection, replacing metronidazole which was previously first-line for mild cases. Fidaxomicin is an alternative, particularly for recurrent episodes.

A patient presents with a painless genital ulcer. Dark-field microscopy of the ulcer exudate shows spirochaetes. What is the recommended treatment?

Answer: Intramuscular benzathine penicillin G (single dose)

A painless genital ulcer (chancre) with spirochaetes on dark-field microscopy is primary syphilis caused by Treponema pallidum. First-line treatment is a single intramuscular dose of benzathine penicillin G 2.4 million units. Doxycycline for 14 days is the alternative for penicillin-allergic patients.