Exam Prep
Lab methods, intestinal and blood protozoa, nematodes, cestodes and trematodes, with clinical correlation.
Independent practice questions written by Labologics. Not affiliated with DHA, DOH (formerly HAAD), SCFHS, Prometric or any exam body. Not recalled exam questions.
For a complete ova and parasite (O&P) examination, how many stool specimens are usually recommended?
Answer: A — Three, collected on separate days
Parasites are shed intermittently. Examination includes a direct wet mount, a concentration (formalin-ethyl acetate) and a permanent stain (e.g., trichrome).
Reference: Garcia: Diagnostic Medical Parasitology
Iodine added to a stool wet mount helps to:
Answer: B — Show nuclei and glycogen of protozoan cysts
Iodine kills trophozoites, so motility must be seen in a saline mount.
Reference: Garcia: Diagnostic Medical Parasitology
Which permanent stain is commonly used to identify intestinal protozoa in stool?
Answer: C — Trichrome
Modified acid-fast stains are used for Cryptosporidium, Cyclospora and Cystoisospora.
Reference: Garcia: Diagnostic Medical Parasitology
For malaria diagnosis, the thick film is used to ___ and the thin film to ___.
Answer: D — Detect parasites / identify species
Giemsa stain at pH 7.2 is used. Films should be repeated every 12-24 hours (at least 3 sets) if negative and malaria is still suspected.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Blood for Wuchereria bancrofti microfilariae should be collected:
Answer: A — At night, around 10 pm to 2 am
W. bancrofti shows nocturnal periodicity. Its microfilariae are sheathed.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Perianal itching in a child at night. The best sample is:
Answer: B — Cellophane (Scotch) tape preparation from the perianal area in the early morning
Enterobius vermicularis females lay eggs on the perianal skin at night. Eggs are oval and flattened on one side.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Which test samples duodenal contents for Giardia when stool tests are repeatedly negative?
Answer: C — String test (Entero-Test)
Stool antigen tests and molecular panels are also highly sensitive.
Reference: Garcia: Diagnostic Medical Parasitology
Bloody dysentery with trophozoites containing ingested red cells suggests:
Answer: D — Entamoeba histolytica
E. histolytica can also cause amoebic liver abscess. Cysts have up to 4 nuclei and cigar-shaped chromatoidal bars.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Why can't microscopy alone separate E. histolytica from E. dispar?
Answer: A — They look identical; antigen tests or PCR are needed
E. dispar is non-pathogenic. Only trophozoites with ingested red cells suggest E. histolytica.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Pear-shaped trophozoites with two nuclei ('face-like'), a sucking disc and falling-leaf motility identify:
Answer: B — Giardia duodenalis (lamblia)
Giardia spreads through contaminated water and food and causes fatty, foul-smelling diarrhea.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Small (4-6 um) acid-fast oocysts in the stool of an AIDS patient with chronic watery diarrhea indicate:
Answer: C — Cryptosporidium
Cyclospora oocysts are larger (8-10 um), variably acid-fast and autofluorescent. Cryptosporidium resists chlorine and spreads in swimming pools.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
The only ciliate that infects humans, linked to contact with pigs, is:
Answer: D — Balantidium coli
It is the largest intestinal protozoan.
Reference: Garcia: Diagnostic Medical Parasitology
Frothy yellow-green vaginal discharge with motile flagellates showing jerky motility suggests:
Answer: A — Trichomonas vaginalis
NAAT is the most sensitive test; wet mounts must be read within minutes. Fishy odor with clue cells suggests bacterial vaginosis (Gardnerella).
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Many small ring forms, multiple rings per cell, and crescent (banana)-shaped gametocytes indicate:
Answer: B — Plasmodium falciparum
Infected red cells are not enlarged. P. falciparum causes the most severe disease, so report it urgently with % parasitemia.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Enlarged infected red cells with Schuffner's dots and amoeboid trophozoites suggest:
Answer: C — Plasmodium vivax
P. ovale also shows Schuffner's dots, with oval, fimbriated red cells. P. vivax and P. ovale form hypnozoites, which cause relapses.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Malaria is transmitted to humans by:
Answer: D — Female Anopheles mosquito
Humans are intermediate hosts; sexual reproduction occurs in the mosquito.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Intraerythrocytic rings with tetrads ('Maltese cross'), no pigment and no gametocytes, after a tick bite, suggest:
Answer: A — Babesia
Babesia is transmitted by Ixodes ticks and by transfusion.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Leishmaniasis is transmitted by:
Answer: B — Sandfly
L. donovani causes visceral leishmaniasis (kala-azar); L. tropica and L. major cause cutaneous leishmaniasis (oriental sore), which is common in parts of Pakistan.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
In a bone marrow or skin smear, Leishmania appears as:
Answer: C — Amastigotes (LD bodies) inside macrophages
Amastigotes show a nucleus and a rod-shaped kinetoplast. Promastigotes are seen in culture and in the sandfly.
Reference: Garcia: Diagnostic Medical Parasitology
Chagas disease is caused by:
Answer: D — Trypanosoma cruzi
T. cruzi is spread by reduviid (kissing) bugs in the Americas. T. brucei causes African sleeping sickness and is spread by tsetse flies.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
The definitive host of Toxoplasma gondii is the:
Answer: A — Cat
Humans are intermediate hosts, infected by oocysts from cat feces or cysts in undercooked meat. Primary infection in pregnancy can cause congenital toxoplasmosis.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
A child develops rapidly fatal meningoencephalitis days after swimming in warm fresh water. Motile amoebae are seen in CSF. The organism is:
Answer: B — Naegleria fowleri
It enters through the nose. Acanthamoeba causes keratitis (contact lens users) and granulomatous amoebic encephalitis.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Fertilized eggs with a thick, bumpy (mammillated) outer shell in stool belong to:
Answer: C — Ascaris lumbricoides
Ascaris is the largest intestinal roundworm; larvae migrate through the lungs.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Barrel-shaped, yellow-brown eggs with polar plugs at both ends belong to:
Answer: D — Trichuris trichiura (whipworm)
Heavy infection in children can cause rectal prolapse and dysentery.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Which intestinal helminth most often causes iron deficiency anemia and positive occult blood?
Answer: A — Hookworm (Necator, Ancylostoma)
Adults attach to the small intestinal wall and suck blood. Larvae penetrate bare skin.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Rhabditiform LARVAE (not eggs) in fresh stool are characteristic of:
Answer: B — Strongyloides stercoralis
Autoinfection can cause life-threatening hyperinfection in patients on corticosteroids or with HTLV-1. Migrating larvae cause 'larva currens' rash.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
A creeping, itchy, serpiginous skin track after walking barefoot on a beach is usually caused by:
Answer: C — Dog or cat hookworm larvae (cutaneous larva migrans)
Humans are accidental hosts, so the larvae cannot complete their life cycle.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Fever, muscle pain, periorbital edema, eosinophilia and high CK after eating undercooked pork suggest:
Answer: D — Trichinella spiralis
Adults live in the intestine; larvae encyst in skeletal muscle. Diagnose with serology or muscle biopsy.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Which feature separates Taenia solium from Taenia saginata?
Answer: A — T. solium has an armed scolex (hooks) and gravid proglottids with fewer lateral uterine branches
The eggs of the two species are identical. T. solium (pork) eggs can also cause cysticercosis, including neurocysticercosis.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Hydatid cyst disease in humans is caused by:
Answer: B — Echinococcus granulosus
Dogs are definitive hosts; humans are infected by swallowing eggs from dog feces. Diagnose with imaging and serology (ELISA, IHA).
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
A cyst removed at surgery is examined in the lab. Which finding confirms hydatid disease?
Answer: C — Protoscolices and hooklets ('hydatid sand') in the cyst fluid
Serology supports the diagnosis before surgery. The cyst fluid is antigenic, and spillage can cause anaphylaxis.
Reference: Garcia: Diagnostic Medical Parasitology
The dwarf tapeworm, which can complete its life cycle within one human host, is:
Answer: D — Hymenolepis nana
Its eggs have polar filaments between the inner and outer membranes.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Eggs with a TERMINAL spine found in urine (with hematuria) belong to:
Answer: A — Schistosoma haematobium
S. haematobium is linked to bladder cancer. Collect urine around midday, ideally after exercise.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Eggs with a prominent LATERAL spine in stool belong to:
Answer: B — Schistosoma mansoni
S. japonicum eggs have a small lateral knob.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Fasciola hepatica, acquired by eating watercress, is a:
Answer: C — Liver fluke
It migrates through the liver into the bile ducts.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Which white cell typically rises in tissue-invasive helminth infections?
Answer: D — Eosinophils
Eosinophilia is less marked in protozoan infections.
Reference: Garcia: Diagnostic Medical Parasitology
Which intestinal parasite causes severe chronic diarrhea in people with AIDS?
Answer: A — Cryptosporidium
Cystoisospora, Cyclospora and microsporidia are other opportunistic causes.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern
Which parasite is transmitted by the tsetse fly?
Answer: B — Trypanosoma brucei
Tsetse fly transmits African sleeping sickness; reduviid bugs transmit T. cruzi; sandflies transmit Leishmania.
Reference: CDC DPDx: Laboratory Identification of Parasites of Public Health Concern