A 27-year-old male who works on an organic farm is diagnosed with infection by N. americanus, a helminthic parasite. Eosinophils require which antibody isotype to destroy these parasites via antibody-dependent cellular cytotoxicity?
Q2
A 29-year-old man presents to the clinic with several days of flatulence and greasy, foul-smelling diarrhea. He says that he was on a camping trip last week after which his symptoms started. When asked further about his camping activities, he reports collecting water from a stream but did not boil or chemically treat the water. The patient also reports nausea, weight loss, and abdominal cramps followed by sudden diarrhea. He denies tenesmus, urgency, and bloody diarrhea. His temperature is 37°C (98.6° F), respiratory rate is 15/min, pulse is 107/min, and blood pressure is 89/58 mm Hg. A physical examination is performed where nothing significant was found except for dry mucous membranes. Intravenous fluids are started and a stool sample is sent to the lab, which reveals motile protozoa on microscopy, negative for any ova, no blood cells, and pus cells. What is the most likely diagnosis?
Q3
A 34-year-old woman with HIV comes to the emergency department because of a 2-week history of diarrhea and abdominal cramping. She has had up to 10 watery stools per day. She also has anorexia and nausea. She returned from a trip to Mexico 4 weeks ago where she went on two hiking trips and often drank from spring water. She was diagnosed with HIV 12 years ago. She says that she has been noncompliant with her therapy. Her last CD4+ T-lymphocyte count was 85/mm3. She appears thin. She is 175 cm (5 ft 9 in) tall and weighs 50 kg (110 lb); BMI is 16.3 kg/m2. Her temperature is 38.3°C (100.9°F), pulse is 115/min, and blood pressure is 85/65 mm Hg. Examination shows dry mucous membranes. The abdomen is soft, and there is diffuse tenderness to palpation with no guarding or rebound. Bowel sounds are hyperactive. Microscopy of a modified acid-fast stain on a stool sample reveals oocysts. Which of the following is the most likely causal organism?
Q4
A 31-year-old man comes to the physician because of a 2-day history of abdominal pain and diarrhea. He reports that his stools are streaked with blood and mucus. He returned from a vacation in the Philippines 3 weeks ago. His vital signs are within normal limits. Abdominal examination shows hyperactive bowel sounds. A photomicrograph of a trichrome-stained wet mount of a stool specimen is shown. Which of the following organisms is the most likely cause of this patient's symptoms?
Q5
A 62-year-old man is referred to a gastroenterologist because of difficulty swallowing for the past 5 months. He has difficulty swallowing both solid and liquid foods, but there is no associated pain. He denies any shortness of breath or swelling in his legs. He immigrated from South America 10 years ago. He is a non-smoker and does not drink alcohol. His physical examination is unremarkable. A barium swallow study was ordered and the result is given below. Esophageal manometry confirms the diagnosis. What is the most likely underlying cause of this patient’s condition?
Q6
A 48-year-old man presents to the clinic with several weeks of watery diarrhea and right upper quadrant pain with fever. He also endorses malaise, nausea, and anorexia. He is HIV-positive and is currently on antiretroviral therapy. He admits to not being compliant with his current medications. His temperature is 37°C (98.6°F), respiratory rate is 15/min, pulse is 70/min, and blood pressure is 100/84 mm Hg. A physical examination is performed which is within normal limits. His blood tests results are given below:
Hb%: 11 gm/dL
Total count (WBC): 3,400 /mm3
Differential count:
Neutrophils: 70%
Lymphocytes: 25%
Monocytes: 5%
CD4+ cell count: 88/mm3
Stool microscopy results are pending. What is the most likely diagnosis?
Q7
A 48-year-old man from Argentina presents to your office complaining of difficulty swallowing for the past few months. He is accompanied by his wife who adds that his breath has started to smell horrible. The patient says that he feels uncomfortable no matter what he eats or drinks. He also has lost 5 kg (11 lb) in the last 2 months. The patient is afebrile, and his vital signs are within normal limits. Physical exam is unremarkable. A barium swallow study along with esophageal manometry is performed and the results are shown in the image below. Manometry shows very high pressure at the lower esophageal sphincter. Which of the following is the most likely etiology of this patient’s symptoms?
Q8
A 36-year-old man is brought to the emergency department for right upper quadrant abdominal pain that began 3 days ago. The pain is nonradiating and has no alleviating or exacerbating factors. He denies any nausea or vomiting. He immigrated from Mexico 6 months ago and currently works at a pet shop. He has been healthy except for 1 week of bloody diarrhea 5 months ago. He is 182 cm (5 ft 11 in) tall and weighs 120 kg (264 lb); BMI is 36 kg/m2. His temperature is 101.8°F (38.8°C), pulse is 85/min, respirations are 14/min, and blood pressure is 120/75 mm Hg. Lungs are clear to auscultation. He has tenderness to palpation in the right upper quadrant. Laboratory studies show:
Hemoglobin 11.7 g/dL3
Leukocyte Count 14,000/mm
Segmented neutrophils 74%
Eosinophils 2%
Lymphocytes 17%
Monocytes 7%
Platelet count 140,000/mm3
Serum
Na+ 139 mEq/L
Cl- 101 mEq/L
K+ 4.4 mEq/L
HCO3- 25 mEq/L
Urea nitrogen 8 mg/dL
Creatinine 1.6 mg/dL
Total bilirubin 0.4 mg/dL
AST 76 U/L
ALT 80 U/L
Alkaline phosphatase 103 U/L
Ultrasonography of the abdomen shows a 4-cm round, hypoechoic lesion in the right lobe of the liver with low-level internal echoes. Which of the following is the most likely diagnosis?
Q9
A 26-year-old woman comes to the physician because of several days of fever, abdominal cramps, and diarrhea. She drank water from a stream 1 week ago while she was hiking in the woods. Abdominal examination shows increased bowel sounds. Stool analysis for ova and parasites shows flagellated multinucleated trophozoites. Further evaluation shows the presence of antibodies directed against the pathogen. Secretion of these antibodies most likely requires binding of which of the following?
Q10
A 30-year-old Caucasian male is brought to the emergency room for recurrent diarrhea. He has had multiple upper respiratory infections since birth and does not take any medications at home. It is determined that Giardia lamblia is responsible for the recurrent diarrhea. The physician performs a serum analysis and finds normal levels of mature B lymphocytes. What other finding on serum analysis predisposes the patient to recurrent diarrheal infections?
Intestinal protozoa (Giardia, Entamoeba) US Medical PG Practice Questions and MCQs
Question 1: A 27-year-old male who works on an organic farm is diagnosed with infection by N. americanus, a helminthic parasite. Eosinophils require which antibody isotype to destroy these parasites via antibody-dependent cellular cytotoxicity?
A. IgE (Correct Answer)
B. IgA
C. IgG
D. IgM
E. IgD
Explanation: ***IgE***
- **IgE** antibodies are crucial in the immune response against helminthic parasites, including *N. americanus*, by sensitizing **mast cells** and **eosinophils**.
- When **IgE** binds to the surface of parasites, the **Fc receptor** on eosinophils recognizes the Fc portion of IgE, leading to the release of cytotoxic granules that destroy the parasite (antibody-dependent cellular cytotoxicity).
*IgA*
- **IgA** is primarily found in **mucosal secretions** and plays a role in defending against pathogens at mucosal surfaces, but it is not the primary isotype involved in eosinophil-mediated **ADCC** against helminths.
- While IgA can bind to some immune cells, its main function is to **neutralize toxins** and prevent microbial adhesion at mucosal sites.
*IgG*
- **IgG** is the most abundant antibody in serum and is involved in various immune functions, including **opsonization**, **neutralization**, and **complement activation**.
- Although IgG can mediate ADCC by **NK cells** and **macrophages**, it is not the primary isotype for eosinophil-mediated killing of helminths, which is dominated by IgE.
*IgM*
- **IgM** is typically the first antibody produced during a primary immune response and is very effective at **activating the complement system**.
- Its large pentameric structure also limits its diffusion into tissues, and it does not play a direct role in eosinophil-mediated ADCC against helminthic parasites.
*IgD*
- **IgD** primarily functions as a **B cell receptor** on naive B lymphocytes, signaling for their activation and differentiation.
- It is present in very low concentrations in serum and its role in host defense against parasites or in ADCC is negligible.
Question 2: A 29-year-old man presents to the clinic with several days of flatulence and greasy, foul-smelling diarrhea. He says that he was on a camping trip last week after which his symptoms started. When asked further about his camping activities, he reports collecting water from a stream but did not boil or chemically treat the water. The patient also reports nausea, weight loss, and abdominal cramps followed by sudden diarrhea. He denies tenesmus, urgency, and bloody diarrhea. His temperature is 37°C (98.6° F), respiratory rate is 15/min, pulse is 107/min, and blood pressure is 89/58 mm Hg. A physical examination is performed where nothing significant was found except for dry mucous membranes. Intravenous fluids are started and a stool sample is sent to the lab, which reveals motile protozoa on microscopy, negative for any ova, no blood cells, and pus cells. What is the most likely diagnosis?
A. Traveler’s diarrhea due to ETEC
B. Irritable bowel syndrome
C. C. difficile colitis
D. Giardiasis (Correct Answer)
E. Traveler’s diarrhea due to Norovirus
Explanation: ***Giardiasis***
- The patient's history of drinking untreated stream water after a camping trip, followed by **greasy, foul-smelling diarrhea**, flatulence, nausea, and weight loss, is highly characteristic of **giardiasis**.
- The stool microscopy finding of **motile protozoa** without blood cells or pus cells confirms the presence of *Giardia lamblia* infection, which is a common cause of such symptoms in campers.
*Traveler’s diarrhea due to ETEC*
- **ETEC (Enterotoxigenic *Escherichia coli*)** typically causes watery diarrhea, often without the prominent greasy, foul-smelling, and chronic nature seen in this case.
- While ETEC is a common cause of traveler's diarrhea, the **microscopic finding of motile protozoa** in the stool definitively rules out a bacterial cause like ETEC.
*Irritable bowel syndrome*
- **Irritable bowel syndrome (IBS)** is a chronic functional disorder of the bowel, characterized by abdominal pain, bloating, and altered bowel habits (diarrhea, constipation, or both), but it does not have an acute onset directly linked to water consumption or involve **motile protozoa** in the stool.
- IBS is a diagnosis of exclusion and would not be considered in the presence of a clear infectious etiology identified by stool examination.
*C. difficile colitis*
- **Clostridioides difficile colitis** typically presents with watery to bloody diarrhea, abdominal pain, and fever, usually following **antibiotic use** or in hospitalized patients, none of which are reported here.
- The stool microscopy would show evidence of *C. difficile* toxins, not **motile protozoa**, and would often reveal pus cells or inflammatory markers.
*Traveler’s diarrhea due to Norovirus*
- **Norovirus** typically causes acute onset of vomiting, watery diarrhea, and abdominal cramps, often resolving within a few days, but it is a **viral infection**.
- The detection of **motile protozoa** on stool microscopy rules out a viral etiology like Norovirus.
Question 3: A 34-year-old woman with HIV comes to the emergency department because of a 2-week history of diarrhea and abdominal cramping. She has had up to 10 watery stools per day. She also has anorexia and nausea. She returned from a trip to Mexico 4 weeks ago where she went on two hiking trips and often drank from spring water. She was diagnosed with HIV 12 years ago. She says that she has been noncompliant with her therapy. Her last CD4+ T-lymphocyte count was 85/mm3. She appears thin. She is 175 cm (5 ft 9 in) tall and weighs 50 kg (110 lb); BMI is 16.3 kg/m2. Her temperature is 38.3°C (100.9°F), pulse is 115/min, and blood pressure is 85/65 mm Hg. Examination shows dry mucous membranes. The abdomen is soft, and there is diffuse tenderness to palpation with no guarding or rebound. Bowel sounds are hyperactive. Microscopy of a modified acid-fast stain on a stool sample reveals oocysts. Which of the following is the most likely causal organism?
A. Entamoeba histolytica
B. Cytomegalovirus
C. Cryptosporidium parvum (Correct Answer)
D. Giardia lamblia
E. Rotavirus
Explanation: ***Cryptosporidium parvum***
- This patient's **immunosuppressed state** (HIV with CD4 count of 85/mm3), severe **watery diarrhea**, and history of drinking **spring water** are highly suggestive of cryptosporidiosis.
- The finding of **oocysts** on modified acid-fast stain is the definitive diagnostic test for *Cryptosporidium*.
*Entamoeba histolytica*
- While *Entamoeba histolytica* can cause diarrhea, it typically presents with **bloody stools (dysentery)** and may cause **liver abscesses**, which are not described here.
- Diagnosis involves identifying trophozoites or cysts in stool, not acid-fast oocysts.
*Cytomegalovirus*
- **CMV colitis** can cause diarrhea in immunocompromised patients, but it is typically diagnosed by **biopsy showing inclusions** and is not characterized by acid-fast oocysts in stool.
- CMV often presents with systemic symptoms, and while abdominal pain is common, the specific stool findings point elsewhere.
*Giardia lamblia*
- *Giardia lamblia* causes **greasy, foul-smelling stools** and **malabsorption**, often without fever, and is diagnosed by finding **cysts or trophozoites** in stool, which are not acid-fast oocysts.
- While acquired from contaminated water, the clinical picture and diagnostic test differ markedly.
*Rotavirus*
- **Rotavirus** is a common cause of severe watery diarrhea, especially in **infants and young children**, but it is less common in adults without specific risk factors and is not diagnosed by acid-fast oocysts.
- It does not specifically target immunocompromised individuals like *Cryptosporidium* does in this context.
Question 4: A 31-year-old man comes to the physician because of a 2-day history of abdominal pain and diarrhea. He reports that his stools are streaked with blood and mucus. He returned from a vacation in the Philippines 3 weeks ago. His vital signs are within normal limits. Abdominal examination shows hyperactive bowel sounds. A photomicrograph of a trichrome-stained wet mount of a stool specimen is shown. Which of the following organisms is the most likely cause of this patient's symptoms?
A. Campylobacter jejuni
B. Giardia lamblia
C. Cryptosporidium parvum
D. Shigella dysenteriae
E. Entamoeba histolytica (Correct Answer)
Explanation: ***Entamoeba histolytica***
- The patient's symptoms of **bloody, mucoid diarrhea** (dysentery) after travel to the **Philippines** are highly suggestive of **amebic dysentery** caused by *Entamoeba histolytica*.
- A **trichrome-stained wet mount** of stool revealing trophozoites or cysts of *Entamoeba histolytica* in this clinical context would confirm the diagnosis.
*Campylobacter jejuni*
- While *Campylobacter jejuni* can cause **bloody diarrhea** and is a common cause of **traveler's diarrhea**, it is a **bacterial infection**, and the question implies a parasitic etiology with the mention of a trichrome-stained wet mount.
- Infection is typically associated with consumption of undercooked poultry or contaminated water.
*Giardia lamblia*
- *Giardia lamblia* causes **giardiasis**, which typically presents with **non-bloody, watery diarrhea**, **steatorrhea**, abdominal cramps, and bloating.
- It does not usually cause **dysentery** or bloody stools.
*Cryptosporidium parvum*
- *Cryptosporidium parvum* causes **cryptosporidiosis**, characterized by **profuse, watery diarrhea** and abdominal cramps.
- While it can be severe in immunocompromised individuals, it typically does not cause **bloody or mucoid stools** (dysentery).
*Shigella dysenteriae*
- *Shigella dysenteriae* causes **shigellosis**, a severe form of dysentery with **bloody, mucoid stools**, fever, and tenesmus.
- While it fits the clinical picture of dysentery, the diagnostic method mentioned (trichrome-stained wet mount) is primarily used for identifying **parasitic organisms**, not bacteria like *Shigella*.
Question 5: A 62-year-old man is referred to a gastroenterologist because of difficulty swallowing for the past 5 months. He has difficulty swallowing both solid and liquid foods, but there is no associated pain. He denies any shortness of breath or swelling in his legs. He immigrated from South America 10 years ago. He is a non-smoker and does not drink alcohol. His physical examination is unremarkable. A barium swallow study was ordered and the result is given below. Esophageal manometry confirms the diagnosis. What is the most likely underlying cause of this patient’s condition?
A. Pharyngoesophageal diverticulum
B. Chagas disease (Correct Answer)
C. Esophageal rupture
D. Gastroesophageal reflux disease
E. Squamous cell carcinoma of the esophagus
Explanation: ***Chagas disease***
- The patient's history of living in **South America** and presenting with **dysphagia for both solids and liquids** (suggesting a motility disorder), along with the **barium swallow image showing esophageal dilation and a 'bird's beak' appearance** at the gastroesophageal junction, are highly characteristic of achalasia caused by Chagas disease.
- Chagas disease, caused by *Trypanosoma cruzi*, leads to the destruction of **myenteric plexus neurons** in the esophagus, resulting in achalasia (failure of the lower esophageal sphincter to relax) and megaesophagus.
*Pharyngoesophageal diverticulum*
- This typically presents as **Zenker's diverticulum**, causing **difficulty initiating a swallow**, regurgitation of undigested food, and sometimes halitosis, which is different from the described dysphagia for both solids and liquids.
- A Zenker's diverticulum would appear as a **pouch-like protrusion** in the posterior pharynx, not the diffuse esophageal dilation seen in the image.
*Esophageal rupture*
- Esophageal rupture (Boerhaave syndrome) is an acute, life-threatening condition associated with **severe chest pain, vomiting, and crepitus**, not a chronic, progressive dysphagia without pain.
- Imaging would reveal **extravasation of contrast** into the mediastinum or pleural space, not the smooth dilation and distal narrowing observed.
*Gastroesophageal reflux disease*
- While chronic GERD can lead to **strictures** and dysphagia, it typically causes **heartburn**, regurgitation, and sometimes odynophagia, and the dysphagia is usually progressive for solids first.
- The barium swallow would show reflux or a stricture, not the **classic achalasia findings** of a dilated esophagus tapering to a narrow distal segment.
*Squamous cell carcinoma of the esophagus*
- Squamous cell carcinoma usually presents with **progressive dysphagia, initially for solids**, and is often associated with weight loss, smoking, and alcohol use, none of which are present in this patient.
- A tumor would typically appear as an **irregular, focal narrowing or mass** on barium swallow, not the smooth, diffuse dilation seen in this image.
Question 6: A 48-year-old man presents to the clinic with several weeks of watery diarrhea and right upper quadrant pain with fever. He also endorses malaise, nausea, and anorexia. He is HIV-positive and is currently on antiretroviral therapy. He admits to not being compliant with his current medications. His temperature is 37°C (98.6°F), respiratory rate is 15/min, pulse is 70/min, and blood pressure is 100/84 mm Hg. A physical examination is performed which is within normal limits. His blood tests results are given below:
Hb%: 11 gm/dL
Total count (WBC): 3,400 /mm3
Differential count:
Neutrophils: 70%
Lymphocytes: 25%
Monocytes: 5%
CD4+ cell count: 88/mm3
Stool microscopy results are pending. What is the most likely diagnosis?
A. C. difficile colitis
B. Traveler’s diarrhea due to ETEC
C. Cryptosporidiosis (Correct Answer)
D. Norovirus infection
E. Irritable bowel syndrome
Explanation: ***Cryptosporidiosis***
- This patient's **HIV-positive status** with a **CD4+ count of 88/mm3** indicates severe immunosuppression, making them highly susceptible to opportunistic infections. The combination of **watery diarrhea**, **right upper quadrant pain with fever** (suggestive of biliary involvement), and general malaise is characteristic of cryptosporidiosis in immunocompromised individuals.
- **Cryptosporidium** infection often causes **chronic, severe watery diarrhea** in patients with AIDS, and can also lead to **cholangitis or cholecystitis**, explaining the right upper quadrant pain and fever.
*C. difficile colitis*
- While *C. difficile* can cause watery diarrhea, especially in hospitalized patients or those on antibiotics, the **right upper quadrant pain with fever** and the patient's profound **immunocompromise (CD4+ <200)** makes opportunistic infections like *Cryptosporidium* more likely.
- *C. difficile* typically presents with **colitis**, which may include abdominal pain but less commonly high-grade fever or specific right upper quadrant pain indicative of biliary involvement.
*Traveler’s diarrhea due to ETEC*
- **ETEC (Enterotoxigenic *E. coli*)** is a common cause of traveler's diarrhea, usually self-limiting and associated with recent travel, which is not mentioned here.
- While it causes watery diarrhea, it typically does not present with **fever or right upper quadrant pain** suggestive of biliary disease, especially not in a patient with severe immunosuppression where opportunistic pathogens are more expected.
*Norovirus infection*
- **Norovirus** causes acute gastroenteritis with **vomiting and watery diarrhea**, sometimes low-grade fever, but it is typically a self-limiting illness lasting 1-3 days.
- It does not explain the **right upper quadrant pain with fever** suggestive of biliary involvement, nor the prolonged "several weeks" duration of symptoms.
*Irritable bowel syndrome*
- **Irritable bowel syndrome (IBS)** is a functional gastrointestinal disorder characterized by chronic abdominal pain and altered bowel habits (diarrhea, constipation, or both), but it does not cause **fever, laboratory abnormalities (low CD4 count), or acute onset of symptoms** in an immunocompromised patient.
- IBS is a diagnosis of exclusion and does not account for the systemic symptoms like **fever, malaise**, or the specific right upper quadrant pain.
Question 7: A 48-year-old man from Argentina presents to your office complaining of difficulty swallowing for the past few months. He is accompanied by his wife who adds that his breath has started to smell horrible. The patient says that he feels uncomfortable no matter what he eats or drinks. He also has lost 5 kg (11 lb) in the last 2 months. The patient is afebrile, and his vital signs are within normal limits. Physical exam is unremarkable. A barium swallow study along with esophageal manometry is performed and the results are shown in the image below. Manometry shows very high pressure at the lower esophageal sphincter. Which of the following is the most likely etiology of this patient’s symptoms?
A. Trypanosoma cruzi infection (Correct Answer)
B. Food allergy
C. Outpouching of the mucosa and submucosa
D. Malignant proliferation of squamous cells
E. Pyloric stenosis
Explanation: ***Trypanosoma cruzi infection***
- The patient's origin from **Argentina** (an endemic area for **Chagas disease**), coupled with symptoms of **dysphagia**, **weight loss**, and **halitosis**, strongly suggests **achalasia** secondary to *Trypanosoma cruzi* infection.
- **Esophageal manometry** showing **very high pressure at the lower esophageal sphincter** and **absent or diminished peristalsis** is characteristic of achalasia, which in this context points to **Chagasic achalasia**.
*Food allergy*
- **Food allergies** typically present with acute symptoms such as **hives, angioedema, gastrointestinal upset**, or **anaphylaxis**, rather than chronic dysphagia and weight loss.
- While eosinophilic esophagitis (a type of food allergy) can cause dysphagia, it usually presents with **esophageal strictures** or **rings** and is not associated with the distinct manometric findings of achalasia.
*Outpouching of the mucosa and submucosa*
- An **outpouching of the mucosa and submucosa** refers to a **Zenker's diverticulum**, which can cause dysphagia, halitosis, and regurgitation.
- However, Zenker's diverticulum is typically located in the **upper esophagus** and would not cause increased lower esophageal sphincter pressure on manometry.
*Malignant proliferation of squamous cells*
- A **malignant proliferation of squamous cells** refers to **esophageal squamous cell carcinoma**, which can cause progressive dysphagia and weight loss.
- While possible in an older patient, the characteristic manometric findings (high LES pressure) are more indicative of achalasia rather than a primary obstructive tumor.
*Pyloric stenosis*
- **Pyloric stenosis** is an obstruction at the gastric outlet, causing **projectile vomiting** and **weight loss**, primarily seen in infants.
- In adults, it is usually due to peptic ulcer disease or malignancy and would not cause esophageal symptoms or increased lower esophageal sphincter pressure.
Question 8: A 36-year-old man is brought to the emergency department for right upper quadrant abdominal pain that began 3 days ago. The pain is nonradiating and has no alleviating or exacerbating factors. He denies any nausea or vomiting. He immigrated from Mexico 6 months ago and currently works at a pet shop. He has been healthy except for 1 week of bloody diarrhea 5 months ago. He is 182 cm (5 ft 11 in) tall and weighs 120 kg (264 lb); BMI is 36 kg/m2. His temperature is 101.8°F (38.8°C), pulse is 85/min, respirations are 14/min, and blood pressure is 120/75 mm Hg. Lungs are clear to auscultation. He has tenderness to palpation in the right upper quadrant. Laboratory studies show:
Hemoglobin 11.7 g/dL3
Leukocyte Count 14,000/mm
Segmented neutrophils 74%
Eosinophils 2%
Lymphocytes 17%
Monocytes 7%
Platelet count 140,000/mm3
Serum
Na+ 139 mEq/L
Cl- 101 mEq/L
K+ 4.4 mEq/L
HCO3- 25 mEq/L
Urea nitrogen 8 mg/dL
Creatinine 1.6 mg/dL
Total bilirubin 0.4 mg/dL
AST 76 U/L
ALT 80 U/L
Alkaline phosphatase 103 U/L
Ultrasonography of the abdomen shows a 4-cm round, hypoechoic lesion in the right lobe of the liver with low-level internal echoes. Which of the following is the most likely diagnosis?
A. Hepatic hydatid cyst
B. Liver hemangioma
C. Pyogenic liver abscess
D. Hepatocellular carcinoma
E. Amebiasis (Correct Answer)
Explanation: ***Amebiasis***
- The patient's history of recent immigration from Mexico, prior **bloody diarrhea**, and current presentation with **right upper quadrant pain**, fever, leukocytosis, and an **abscess-like lesion in the liver** are highly suggestive of an **amoebic liver abscess**.
- **Entamoeba histolytica**, often acquired through contaminated food or water, can cause colitis leading to bloody diarrhea and subsequently spread hematogenously to the liver.
*Hepatic hydatid cyst*
- **Hydatid cysts** (Echinococcosis) are typically slow-growing and often asymptomatic for many years before causing symptoms.
- While they can be found in the liver, the presentation with **acute fever** and **leukocytosis** is less common than with an abscess.
*Liver hemangioma*
- A **liver hemangioma** is a benign vascular tumor, usually asymptomatic and discovered incidentally.
- It does not cause fever, elevated white blood cell count, or abdominal pain in this acute manner.
*Pyogenic liver abscess*
- While a **pyogenic liver abscess** can present with similar symptoms (fever, RUQ pain, leukocytosis) and imaging findings, the history of **bloody diarrhea** and immigration from an endemic area points more specifically to amoebic etiology.
- Pyogenic abscesses are often associated with other underlying conditions such as biliary tract disease or diverticulitis.
*Hepatocellular carcinoma*
- **Hepatocellular carcinoma (HCC)** typically occurs in patients with underlying chronic liver disease (e.g., cirrhosis, hepatitis B or C).
- While it can present with abdominal pain and a liver mass, an acute febrile illness and a history of bloody diarrhea are not typical features of HCC.
Question 9: A 26-year-old woman comes to the physician because of several days of fever, abdominal cramps, and diarrhea. She drank water from a stream 1 week ago while she was hiking in the woods. Abdominal examination shows increased bowel sounds. Stool analysis for ova and parasites shows flagellated multinucleated trophozoites. Further evaluation shows the presence of antibodies directed against the pathogen. Secretion of these antibodies most likely requires binding of which of the following?
A. CD28 to B7 protein
B. CD80/86 to CTLA-4
C. gp120 to CD4
D. CD8 to MHC I
E. CD40 to CD40 ligand (Correct Answer)
Explanation: ***CD40 to CD40 ligand***
- The interaction between **CD40 on B cells** and **CD40 ligand (CD40L) on activated T helper cells** is crucial for **T cell-dependent B cell activation** and antibody class switching.
- This binding leads to the maturation of the immune response, including the secretion of **high-affinity antibodies** like IgA, which is especially important for mucosal immunity against pathogens like *Giardia lamblia* (the likely cause of the patient's symptoms).
*CD28 to B7 protein*
- The binding of **CD28 on T cells** to **B7 protein (CD80/86) on antigen-presenting cells (APCs)** provides the **second co-stimulatory signal** required for T cell activation.
- While essential for T cell activation, this interaction primarily supports T cell proliferation and differentiation, rather than directly mediating antibody secretion by B cells.
*CD80/86 to CTLA-4*
- **CTLA-4 (cytotoxic T-lymphocyte-associated protein 4)** is a receptor on T cells that binds to **CD80/86 (B7)** on APCs with higher affinity than CD28.
- This interaction provides an **inhibitory signal** that downregulates T cell activation, serving as a negative feedback mechanism, and does not promote antibody secretion.
*gp120 to CD4*
- The **gp120 glycoprotein** on the surface of **HIV** binds to the **CD4 receptor** on T helper cells, initiating the entry of the virus into the cell.
- This interaction is specific to HIV infection and is not involved in the normal process of antibody secretion in response to other pathogens.
*CD8 to MHC I*
- **CD8** is a co-receptor expressed on **cytotoxic T lymphocytes (CTLs)** that binds to **MHC class I molecules** on target cells.
- This interaction is essential for the recognition of virally infected or cancerous cells by CTLs, leading to their destruction, but it is not directly involved in antibody production.
Question 10: A 30-year-old Caucasian male is brought to the emergency room for recurrent diarrhea. He has had multiple upper respiratory infections since birth and does not take any medications at home. It is determined that Giardia lamblia is responsible for the recurrent diarrhea. The physician performs a serum analysis and finds normal levels of mature B lymphocytes. What other finding on serum analysis predisposes the patient to recurrent diarrheal infections?
A. Deficiency in IgG
B. Deficiency in neutrophils
C. Deficiency in IgA (Correct Answer)
D. Deficiency in NK cells
E. Deficiency in CD8+ T cells
Explanation: ***Deficiency in IgA***
- **Selective IgA deficiency** is the most common primary immunodeficiency and typically presents with recurrent sinopulmonary and gastrointestinal infections.
- **IgA** is crucial for mucosal immunity, and its deficiency renders the patient susceptible to infections like *Giardia lamblia*, which colonizes the gut.
*Deficiency in IgG*
- While IgG deficiency can lead to recurrent infections, it typically involves a broader range of pathogens and often includes more severe bacterial infections.
- The presence of **normal mature B lymphocytes** in this patient suggests a defect in immunoglobulin class switching or secretion rather than a complete absence of antibody-producing cells.
*Deficiency in neutrophils*
- **Neutrophil deficiency** (neutropenia) primarily leads to recurrent bacterial and fungal infections, often presenting with skin abscesses and systemic infections.
- It does not specifically predispose an individual to protozoal infections like *Giardia lamblia* or recurrent upper respiratory tract infections in this manner.
*Deficiency in NK cells*
- **Natural killer (NK) cells** are important for antiviral and antitumor immunity.
- A deficiency in NK cells would typically manifest as increased susceptibility to viral infections (herpesviruses) and certain cancers, not *Giardia lamblia* or recurrent upper respiratory infections, which are often bacterial or viral in origin.
*Deficiency in CD8+ T cells*
- **CD8+ T cells** are crucial for killing virally infected cells and some tumor cells.
- A deficiency would lead to increased susceptibility to severe viral infections and certain intracellular bacteria, not typically chronic *Giardia* infection or recurrent bacterial upper respiratory infections.
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