Infections for NEET PG Pathology: Malaria, Leprosy, Tuberculosis, Typhoid, Amoebiasis, Cysticercosis, Hydatid, COVID-19

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🎯 Exam priority: Moderate–High. Expect: why falciparum is deadly, hypnozoites and relapse, protective red-cell traits, the gold-standard malaria test, tuberculoid vs lepromatous leprosy and the two reactions, Ghon/Ranke complexes, typhoid carriers, amoebic liver abscess, neurocysticercosis and COVID-19 labs.

CBME mapping: PA 10.1–10.5. Granulomas and caseous necrosis are in articles 1–2.

Key pathology and clues for the major infections. Drawn by TrackNation.

Malaria

• Spread by the female Anopheles mosquito (rarely by transfusion, transplant, needles or congenitally). Mosquito stages: gametocyte, oocyst, sporozoite; human stages: schizont, merozoite, trophozoite.

• P. falciparum: infected red cells stick to endothelium → microvascular obstruction and ischaemia of brain, kidneys, lungs and gut. Cerebral malaria: irritability to seizures and coma. Intravascular haemolysis → haemoglobinuria → blackwater fever. Hypoglycaemia is common (worse with quinine). Fever can become tertian (every 48 h).

• P. vivax and P. ovale leave liver hypnozoites that cause relapses years later. People lacking the Duffy blood group resist vivax. Parasite growth is slowed in HbS, HbC, thalassaemia, G6PD deficiency and elliptocytosis.

• Diagnosis in India: test every suspected case at once by microscopy and/or RDT; stained thick and thin smears are the gold standard (thin smear for species and parasite density). Repeat smears 12–24-hourly until three are negative.

Leprosy

Mycobacterium leprae (acid-fast) prefers peripheral nerves, skin and upper airway mucosa: anaesthetic skin lesions and neuropathy. Type 1 (reversal) reactions come from a rise in cell-mediated immunity; type 2 (erythema nodosum leprosum) is a systemic vasculitis/panniculitis. Treatment: dapsone, rifampicin and clofazimine.

Tuberculosis, Typhoid and Others

• TB: a Ghon focus plus involved node = Ghon complex; calcified = Ranke complex; apical scars from seeding = Simon foci. 5–15% of latent infections progress. Lymphadenitis (scrofula) is the commonest extrapulmonary form.

• Typhoid (Salmonella Typhi; humans are the only reservoir): intestinal ulceration, haemorrhage and perforation in severe cases; rose spots in week 2; relative bradycardia. Blood culture is positive in the first 2 weeks, stool culture in weeks 3–5. About 3% of untreated patients become gallbladder carriers.

• Amoebiasis (Entamoeba histolytica): cysts infect, trophozoites invade — flask-shaped colonic ulcers; spread via the portal vein causes liver abscess.

• Cysticercosis: eating T. solium eggs from human faeces → larvae in tissues; in the brain (neurocysticercosis) they cause seizures; scans show cysts, ring-enhancing or calcified lesions. Eating undercooked pork gives the adult tapeworm.

• Hydatid disease (Echinococcus granulosus, sheep-raising areas): cysts in liver or lung; rupture can cause anaphylaxis.

• COVID-19: viral pneumonia with ground-glass opacities; can involve heart, kidney, brain and clotting. Severe disease: lymphopenia with raised AST/ALT, LDH, D-dimer, ferritin and CRP. Diagnosis by antigen test or PCR.

Traps the Examiner Sets

• Only vivax and ovale relapse (hypnozoites); falciparum is the one that blocks capillaries.

• Tuberculoid = strong immunity, few lesions; lepromatous = weak immunity, widespread.

• Reversal reaction = type 1 (CMI up); ENL = type 2.

• Ghon complex calcified = Ranke complex.

• Pork meat → tapeworm; eggs from faeces → cysticercosis.

Memory Hooks & Shortcuts

Shortcuts to lock in this article's numbers and lists. Each one only re-packages facts proven above.

• 🧠 Falciparum is Fatal: it Fastens to endothelium — falciparum cytoadherence

• 🧠 Vivax and Ovale come back in Volleys (hypnozoites) — relapse

• 🧠 Tuberculoid = Tough immunity; Lepromatous = Lax immunity — leprosy spectrum

• 🧠 Ghon goes Rigid (calcified) = Ranke — Ghon vs Ranke

• 🧠 Blood first, Stool later (typhoid) — culture timing

• 🧠 Pork = worm; Poo eggs = cysts in the brain — taeniasis vs cysticercosis

Rapid-Revision Cards

• Malaria vector

  Female Anopheles; also transfusion, transplant, needles, congenital.

• Falciparum danger

  RBCs stick to endothelium → ischaemia (brain, kidney).

• Blackwater fever

  Intravascular haemolysis → dark urine.

• Hypnozoites

  Vivax, ovale → relapse.

• Duffy-negative

  Resist vivax.

• Protective RBC traits

  HbS, HbC, thalassaemia, G6PD, elliptocytosis.

• Malaria test (India)

  Thick + thin smear = gold standard; RDT also.

• Negative smear

  Repeat 12–24 h until 3 negative.

• Tuberculoid vs lepromatous

  Strong vs poor CMI.

• PB vs MB

  ≤ 5 lesions, no bacilli vs ≥ 6 or bacilli.

• Type 1 reaction

  Reversal — CMI rises.

• Type 2 reaction

  ENL — vasculitis/panniculitis.

• Ghon / Ranke

  Focus + node / calcified complex.

• Latent TB

  5–15% progress.

• Typhoid cultures

  Blood weeks 1–2; stool weeks 3–5.

• Typhoid carrier

  Gallbladder, ~3%.

• Amoebic ulcer

  Flask-shaped; liver abscess via portal vein.

• Neurocysticercosis

  T. solium eggs; seizures; ring/calcified lesions.

• Hydatid

  E. granulosus; rupture → anaphylaxis.

• Severe COVID-19 labs

  Lymphopenia; ↑ D-dimer, ferritin, LDH, CRP.

Sources

Facts in this article were checked against these documents.

• MSD Manual Professional: Malaria; Leprosy; Tuberculosis; Typhoid Fever — Merck & Co. (MSD Manuals), reviewed 2024–26 (link)

• MSD Manual Professional: Amebiasis; Taenia solium Infection and Cysticercosis; Echinococcosis; COVID-19 — Merck & Co. (MSD Manuals), reviewed 2024–26 (link)

• Guidelines for Diagnosis and Treatment of Malaria in India — National Vector Borne Disease Control Programme, 2014 (link)

• Competency Based Undergraduate Curriculum, Pathology (PA 10) — National Medical Commission, 2024

Test Yourself: Practice Questions

Thirty questions in NEET PG style, most of them clinical vignettes.

Practice Questions (NEET PG pattern)

Original questions written in the style of the exam: +4 for a correct answer, −1 for a wrong one. Every answer was checked against the sources listed above. NEET PG papers are not published by NBEMS, so these are not past-paper questions.

Q1. Malaria is transmitted to humans mainly by the bite of

• (a) the Culex mosquito

• (b) the female Aedes mosquito

• (c) the sandfly

• (d) the female Anopheles mosquito

Q2. A patient develops malaria weeks after a blood transfusion, with no travel. This shows that malaria can spread by

• (a) blood transfusion

• (b) food

• (c) skin contact

• (d) droplets

Q3. Falciparum malaria causes coma and kidney failure mainly because

• (a) of immune complexes only

• (b) of hypnozoites

• (c) infected red cells stick to vascular endothelium and block microvessels

• (d) the parasite invades neurons

Q4. A patient with falciparum malaria passes very dark urine from intravascular haemolysis. This is

• (a) rhabdomyolysis

• (b) cerebral malaria

• (c) blackwater fever

• (d) porphyria

Q5. A man treated for vivax malaria a year ago has another attack without travelling. The cause is

• (a) re-infection only

• (b) liver hypnozoites

• (c) falciparum resistance

• (d) persistent gametocytes

Q6. Most sub-Saharan Africans are completely resistant to P. vivax because they lack

• (a) the Duffy blood group

• (b) the Rh antigen

• (c) G6PD

• (d) HbS

Q7. Which red-cell condition delays the development of Plasmodium inside red cells?

• (a) iron overload

• (b) polycythaemia vera

• (c) thalassaemia or G6PD deficiency

• (d) spherocytosis only

Q8. A child in a malaria-endemic area develops fever, seizures and coma. The likely complication is

• (a) blackwater fever

• (b) meningitis always

• (c) simple febrile seizure only

• (d) cerebral malaria

Q9. A patient with falciparum malaria is being treated with quinine. A common complication to monitor for is

• (a) hyperkalaemia

• (b) polycythaemia

• (c) hypoglycaemia

• (d) hypercalcaemia

Q10. Under India's malaria guidelines, the gold standard to confirm malaria is

• (a) serology

• (b) a clinical score

• (c) microscopy of stained thick and thin blood smears

• (d) a single RDT

Q11. The first blood smear of a febrile traveller is negative but malaria is still suspected. MSD advises

• (a) repeat smears every 12–24 hours until three are negative

• (b) start treatment without testing

• (c) test only after 1 week

• (d) stop testing

Q12. A man has a few anaesthetic skin lesions; no bacilli are seen in samples. Immunity and classification are

• (a) poor cell-mediated immunity — lepromatous

• (b) strong cell-mediated immunity — tuberculoid, paucibacillary

• (c) multibacillary

• (d) humoral immunity only

Q13. A patient has many skin lesions with acid-fast bacilli in skin samples. The WHO category is

• (a) paucibacillary

• (b) multibacillary

• (c) indeterminate only

• (d) tuberculoid

Q14. Lepromatous leprosy reflects

• (a) no infection of nerves

• (b) a localised single lesion

• (c) strong cell-mediated immunity

• (d) poor cell-mediated immunity with widespread bacterial infiltration

Q15. During treatment, existing leprosy lesions and nerves suddenly become inflamed and tender because cell-mediated immunity rises. This is

• (a) drug allergy

• (b) erythema nodosum leprosum

• (c) relapse

• (d) a type 1 (reversal) reaction

Q16. A patient with lepromatous leprosy develops a systemic inflammatory reaction that behaves like a vasculitis or panniculitis. This is

• (a) erythema nodosum leprosum (type 2 reaction)

• (b) TB co-infection

• (c) type 1 reversal reaction

• (d) tuberculoid conversion

Q17. Mycobacterium leprae has a special tropism for

• (a) lung alveoli only

• (b) bone marrow

• (c) the liver

• (d) peripheral nerves, skin and upper airway mucosa

Q18. A chest X-ray shows a calcified peripheral lung nodule with a calcified hilar node from old primary TB. This is

• (a) a Ghon focus only

• (b) a tuberculoma

• (c) a Ranke complex

• (d) a Simon focus

Q19. Of people with latent TB infection, the proportion who eventually develop active disease is about

• (a) 90%

• (b) 50%

• (c) 5–15%

• (d) less than 0.1%

Q20. A serious intestinal complication that can occur in severe typhoid fever is

• (a) pancreatitis

• (b) intestinal perforation

• (c) appendicitis

• (d) cholecystitis

Q21. A patient has had typhoid fever for 4 days. The culture most likely to be positive is

• (a) stool culture

• (b) blood culture

• (c) urine culture

• (d) bone marrow only after week 4

Q22. A food handler sheds Salmonella Typhi for a year after untreated typhoid. The organisms usually persist in the

• (a) lungs

• (b) kidneys

• (c) gallbladder (about 3% become carriers)

• (d) spleen

Q23. Colonic biopsy in a patient with diarrhoea shows flask-shaped mucosal lesions with trophozoites. The organism and a feared complication are

• (a) Shigella — HUS

• (b) Entamoeba histolytica — liver abscess via the portal vein

• (c) Giardia — malabsorption

• (d) Salmonella — perforation

Q24. The infective form of Entamoeba histolytica is the

• (a) trophozoite

• (b) cyst

• (c) egg

• (d) larva

Q25. A young adult presents with new seizures; CT shows ring-enhancing and calcified cystic brain lesions. The infection is acquired by

• (a) drinking unboiled milk

• (b) eating undercooked beef

• (c) a mosquito bite

• (d) swallowing Taenia solium eggs from human faeces

Q26. A man eats undercooked pork containing cysticerci. The result is

• (a) neurocysticercosis directly

• (b) an adult Taenia solium tapeworm in the intestine

• (c) hydatid cysts

• (d) trichinosis only

Q27. A man from a sheep-raising area has a liver cyst; the cyst ruptures and he develops urticaria and shock. The parasite is

• (a) Fasciola

• (b) Entamoeba histolytica

• (c) Echinococcus granulosus (hydatid cyst)

• (d) Taenia solium

Q28. Laboratory findings typical of severe COVID-19 include

• (a) a low D-dimer

• (b) neutropenia with low ferritin

• (c) eosinophilia

• (d) lymphopenia with raised D-dimer, ferritin, LDH and CRP

Q29. COVID-19 is diagnosed by

• (a) chest X-ray alone

• (b) antibody testing in the first week

• (c) blood culture

• (d) antigen or nucleic acid (PCR) testing of respiratory secretions

Q30. In typhoid, the fever is often accompanied by

• (a) relative bradycardia

• (b) marked tachycardia always

• (c) no splenomegaly ever

• (d) hypothermia

Active Recall

From memory: explain why falciparum malaria is severe and why vivax relapses; list traits protecting against malaria and how India confirms it; compare tuberculoid and lepromatous leprosy with the two reactions; define Ghon and Ranke complexes; give typhoid culture timing and carriage; and describe amoebic, cysticercal and hydatid lesions and COVID-19 labs.