High-Yield Infectious Diseases for Medical Exams

Infectious diseases questions combine microbiology, pharmacology and acute medicine. They reward a clear way of thinking: where is the infection, which organisms are likely, how sick is the patient, and which patient factors change the drug. This guide is exam revision. Antibiotic choices in practice follow local microbiology policy.

The four questions behind every ID stem

1. Source: chest, urine, CNS, skin, abdomen, blood, bone or joint?
2. Likely organisms: shaped by age, setting (community vs hospital), travel, immune status and exposures.
3. Severity: is this sepsis or septic shock?
4. Modifiers: pregnancy, allergy, renal or liver impairment, interacting drugs, recent antibiotics.

Sepsis

Sepsis is life-threatening organ dysfunction caused by a dysregulated response to infection. Septic shock is sepsis with persistent hypotension requiring vasopressors to maintain mean arterial pressure and a raised lactate despite adequate fluid resuscitation.

Early management themes

• Recognise early using a structured score and clinical judgement.
• Take blood cultures before antibiotics, if this does not delay treatment.
• Give broad-spectrum antibiotics promptly; guidelines aim for within one hour in the sickest patients.
• Measure lactate; give intravenous crystalloid for hypotension or high lactate.
• Start vasopressors (noradrenaline first line) if hypotension persists after fluids.
• Find and control the source: drain abscesses, remove infected lines, relieve obstruction.
• Monitor urine output and reassess often.

Exam trap: "next step" in a hypotensive febrile patient is resuscitation and antibiotics, not imaging first.

Pneumonia

Severity scores (such as CURB-65) guide place of care and antibiotic intensity. Mild community-acquired pneumonia is usually treated with amoxicillin in UK guidance, with a macrolide or doxycycline as alternatives; more severe disease adds atypical cover.

Meningitis and encephalitis

CSF patterns

Management themes

• Do not delay antibiotics. Give a third-generation cephalosporin empirically; add amoxicillin or ampicillin where Listeria is possible.
• Dexamethasone is given in suspected pneumococcal meningitis in adults, ideally before or with the first antibiotic dose.
• Perform CT before lumbar puncture only if there are signs of raised intracranial pressure, focal neurology, reduced consciousness or seizures.
• Close contacts of meningococcal disease receive prophylaxis.
• Suspected herpes simplex encephalitis (fever, confusion, temporal lobe signs) needs intravenous aciclovir without waiting for results.

Urinary tract infection

• Lower UTI in non-pregnant women: nitrofurantoin or trimethoprim in many guidelines, chosen by local resistance and renal function.
• Pregnancy: always treat, including asymptomatic bacteriuria; avoid trimethoprim in the first trimester and nitrofurantoin near term.
• Pyelonephritis: fever, loin pain, rigors; needs an agent with good tissue penetration, not nitrofurantoin.
• Catheter-associated: do not treat asymptomatic bacteriuria in catheterised patients.

Tuberculosis

Risk factors: birth or residence in a high-incidence country, HIV, homelessness, prison, close contact, immunosuppression (including anti-TNF therapy).

Standard drug-susceptible pulmonary TB treatment in WHO and national guidance: an intensive phase with four drugs for two months, then a continuation phase with rifampicin and isoniazid. Latent TB is detected with a tuberculin skin test or interferon-gamma release assay and treated to prevent progression.

HIV

• Diagnosis: fourth-generation antigen/antibody tests detect infection earlier than antibody-only tests.
• Treatment: antiretroviral therapy is recommended for everyone with HIV, regardless of CD4 count.
• Opportunistic infections by CD4 count: below 200 cells/mm³, Pneumocystis pneumonia (prophylaxis with co-trimoxazole); lower counts, toxoplasmosis, cryptococcal meningitis, cytomegalovirus retinitis and Mycobacterium avium complex.
• Prevention: pre-exposure prophylaxis for people at high risk; post-exposure prophylaxis started as soon as possible after exposure.
• Pregnancy: antiretroviral therapy greatly reduces vertical transmission.

Malaria

Fever in a returning traveller is malaria until proven otherwise. Plasmodium falciparum causes most severe disease.

• Diagnosis: thick and thin blood films (repeat if negative) or rapid diagnostic tests.
• Uncomplicated falciparum: artemisinin-based combination therapy in WHO guidance.
• Severe malaria (impaired consciousness, high parasitaemia, acidosis, severe anaemia, renal failure): intravenous artesunate.
• Non-falciparum (vivax, ovale): also need treatment of liver hypnozoites with primaquine or tafenoquine after G6PD testing.

Sexually transmitted infections

Partner notification and testing for other STIs, including HIV, are part of the expected answer.

Skin, bone and joint

• Cellulitis: usually streptococci or S. aureus; flucloxacillin in UK guidance.
• Necrotising fasciitis: pain out of proportion to signs, rapid spread; urgent surgical debridement plus broad antibiotics.
• Septic arthritis: hot swollen joint; aspirate before antibiotics if it does not delay treatment; S. aureus is most common; consider gonococcal infection in young sexually active adults.
• Osteomyelitis: MRI is the most sensitive imaging; Salmonella is a classic association in sickle cell disease.

Organism clues that answer questions

Common exam traps

• Delaying antibiotics for imaging or lumbar puncture in suspected meningitis.
• Using nitrofurantoin for pyelonephritis or in severe renal impairment.
• Treating asymptomatic bacteriuria in non-pregnant or catheterised patients.
• Forgetting pyridoxine with isoniazid, or forgetting to check eyes before ethambutol.
• Dismissing a negative single malaria film.
• Missing Listeria in neonates, older adults and pregnancy.
• Forgetting the disulfiram-like reaction of metronidazole with alcohol.

How to revise

Group revision by source of infection, then cross-check with an antibiotic adverse-effect list. Practise mixed vignettes, because exams hide the organism behind exposure clues. The MediFlow Exam Prep app has infectious diseases questions with explanations and re-tests the ones you miss. See also high-yield pharmacology and high-yield respiratory medicine.

Summary

• Think source, organism, severity, modifiers.
• In sepsis and meningitis, early antibiotics matter more than early imaging.
• Know the four TB drugs and their adverse effects.
• Exposure clues in the stem usually name the organism for you.