An 82-year-old woman presents with 2 months of foul-smelling, greasy diarrhea. She says that she also has felt very tired recently and has had some associated bloating and flatus. She denies any recent abdominal pain, nausea, melena, hematochezia, or vomiting. She also denies any history of recent travel and states that her home has city water. Which of the following tests would be most appropriate to initially work up the most likely diagnosis in this patient?

Tissue transglutaminase antibody test

CT of the abdomen with oral contrast

A 27-year-old woman visits her family physician complaining of the recent onset of an unpleasant fish-like vaginal odor that has started to affect her sexual life. She was recently treated for traveler’s diarrhea after a trip to Thailand. External genitalia appear normal on pelvic examination, speculoscopy shows a gray, thin, homogenous, and malodorous vaginal discharge. Cervical mobilization is painless and no adnexal masses are identified. A sample of the vaginal discharge is taken for saline wet mount examination. Which of the following characteristics is most likely to be present in the microscopic evaluation of the sample?

Polymorphonuclear cells (PMNs) to epithelial cell ratio of 2:1

A 45-year-old man visits the office with complaints of severe pain with urination for 5 days. In addition, he reports having burning discomfort and itchiness at the tip of his penis. He is also concerned regarding a yellow-colored urethral discharge that started a week ago. Before his symptoms began, he states that he had sexual intercourse with multiple partners at different parties organized by the hotel he was staying at. Physical examination shows edema and erythema concentrated around the urethral meatus accompanied by a mucopurulent discharge. Which of the following diagnostic tools will best aid in the identification of the causative agent for his symptoms?

Nucleic acid amplification tests (NAATs)

Leukocyte esterase dipstick test

A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1°F (38.9°C), pulse is 56/min, respirations are 16/min, and blood pressure is 150/85 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show: Hemoglobin 13.5 g/dL Leukocyte count 15,000/mm3 Platelet count 130,000/mm3 Serum Na+ 129 mEq/L Cl- 100 mEq/L K+ 4.6 mEq/L HCO3- 22 mEq/L Urea nitrogen 14 mg/dL Creatinine 1.3 mg/dL An x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?

Direct immunofluorescent antibody test

A 22-year-old female presents to her physician for evaluation of a vaginal discharge, itching, and irritation. She recently started a new relationship with her boyfriend, who is her only sexual partner. He does not report any genitourinary symptoms. She takes oral contraceptives and does not use barrier contraception. The medical history is unremarkable. The vital signs are within normal limits. A gynecologic examination reveals a thin, yellow, frothy vaginal discharge with a musty, unpleasant odor and numerous punctate red maculae on the ectocervix. The remainder of the exam is normal. Which of the following organisms will most likely be revealed on wet mount microscopy?

Motile round or oval-shaped microorganisms

Epithelial cells covered by numerous bacterial cells

Budding yeast cells and/or pseudohyphae

A 54-year-old man comes to the physician because of diarrhea that has become progressively worse over the past 4 months. He currently has 4–6 episodes of foul-smelling stools per day. Over the past 3 months, he has had fatigue and a 5-kg (11-lb) weight loss. He returned from Bangladesh 6 months ago after a year-long business assignment. He has osteoarthritis and hypertension. Current medications include amlodipine and naproxen. He appears pale and malnourished. His temperature is 37.3°C (99.1°F), pulse is 76/min, and blood pressure is 140/86 mm Hg. Examination shows pale conjunctivae and dry mucous membranes. Angular stomatitis and glossitis are present. The abdomen is distended but soft and nontender. Rectal examination shows no abnormalities. Laboratory studies show: Hemoglobin 8.9 g/dL Leukocyte count 4100/mm3 Platelet count 160,000/mm3 Mean corpuscular volume 110 μm3 Serum Na+ 133 mEq/L Cl- 98 mEq/l K+ 3.3 mEq/L Creatinine 1.1 mg/dL IgA 250 mg/dL Anti-tissue transglutaminase, IgA negative Stool culture and studies for ova and parasites are negative. Test of the stool for occult blood is negative. Fecal fat content is 22 g/day (N < 7). Fecal lactoferrin is negative and elastase is within normal limits. Which of the following is the most appropriate next step in diagnosis?

PAS-stained biopsy of small bowel

IgG against deamidated gliadin peptide

Laboratory diagnosis of parasites for USMLE Step 2 CK: High-Yield Microbiology Lessons

Microscopy - The Classic Hunt

Direct visualization remains the gold standard for identifying many parasites. The choice of sample and preparation technique is critical.
Stool Examination (Ova & Parasites - O&P):
- Wet Mount (Saline/Iodine): Detects motile trophozoites (e.g., Entamoeba histolytica), cysts, and ova.
- Permanent Stains (e.g., Trichrome): Provides detailed morphology for definitive identification of protozoan cysts and trophozoites.
- Modified Acid-Fast Stain: Crucial for oocysts of Cryptosporidium, Cyclospora, and Cystoisospora.
Blood Smears:
- Giemsa Stain: The standard for blood parasites.
- Used for Plasmodium, Babesia, Trypanosoma, Leishmania, and microfilariae.
⭐ Thick vs. Thin Smear: A thick smear screens for the presence of parasites (higher sensitivity as more blood is viewed). A thin smear allows for species-level identification (e.g., P. falciparum vs. P. vivax) by preserving RBC morphology.

Thick vs. Thin Blood Smears for Malaria Diagnosis

Other Key Preparations:
- CSF Wet Mount: For motile Naegleria fowleri trophozoites.
- Vaginal Wet Mount: For motile Trichomonas vaginalis.
- Muscle Biopsy: For Trichinella larvae.

Advanced Diagnostics - Beyond the Scope

Molecular Methods (NAATs/PCR):
- Highest sensitivity & specificity.
- Detects low parasitemia, speciates organisms (e.g., Plasmodium spp.), & identifies drug resistance genes.
- Key for Leishmania, Toxoplasma, Babesia, Trichomonas.
Antigen Detection:
- Rapid diagnostic tests (RDTs) for stool or blood.
- Examples: Plasmodium HRP2/pLDH, Giardia & Cryptosporidium stool antigen assays.
Serology (Antibody Detection):
- Detects host IgG/IgM; for latent/chronic or tissue stages where organisms are scarce (e.g., Toxoplasma, Strongyloides, T. cruzi, Echinococcus).

⭐ Serology often cannot distinguish between active and past, cured infection, as antibodies can persist for years.

Key Morphologies - Parasite Lineup

Protozoa
- Giardia lamblia: Pear-shaped, flagellated trophozoites; oval cysts.
- Entamoeba histolytica: Trophozoites with ingested RBCs; cysts with up to 4 nuclei.
- Cryptosporidium: Oocysts on acid-fast stain.
- Trypanosoma cruzi: C-shaped trypomastigotes in blood smear.
- Plasmodium: Ring form within RBCs.
Helminths
- Enterobius vermicularis: D-shaped (football-like) eggs on tape test.
- Ascaris lumbricoides: Large, bile-stained, knobby-coated eggs.
- Taenia solium: Scolex with 4 suckers and a circle of hooks.
- Schistosoma: Eggs with characteristic spines (lateral: mansoni, terminal: haematobium).

⭐ Entamoeba histolytica trophozoites containing engulfed red blood cells are pathognomonic.

Morphology of Protozoa and Helminths

Stool O&P is the classic test for most intestinal protozoa and helminths; multiple samples are often required for sensitivity.

Thick and thin blood smears (Giemsa stain) are crucial for diagnosing blood-borne parasites like malaria and trypanosomes.

The Scotch tape test is the preferred method for detecting pinworm (Enterobius vermicularis) eggs.

Serology is key for diagnosing tissue-invasive parasites like Toxoplasma and Echinococcus.

Antigen detection assays offer rapid diagnosis for Giardia and Cryptosporidium.

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