Laboratory diagnosis of parasites US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Laboratory diagnosis of parasites. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Laboratory diagnosis of parasites US Medical PG Question 1: 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?
- A. Fecal fat test (Correct Answer)
- B. Tissue transglutaminase antibody test
- C. Stool O&P
- D. Stool guaiac test
- E. CT of the abdomen with oral contrast
Laboratory diagnosis of parasites Explanation: ***Fecal fat test***
- The patient's symptoms of **foul-smelling, greasy diarrhea**, along with **fatigue, bloating, and flatus**, strongly suggest **malabsorption**, specifically **steatorrhea** (excess fat in stool).
- A **fecal fat test** (e.g., Sudan stain or 72-hour quantitative stool fat collection) directly assesses fat malabsorption and would be the most appropriate initial diagnostic test.
*Tissue transglutaminase antibody test*
- This test is used to screen for **celiac disease**, which can cause malabsorption symptoms.
- While celiac disease is a possibility, a fecal fat test is a more general and appropriate initial step to confirm fat malabsorption before looking for specific causes.
*Stool O&P*
- Stands for **Stool Ova and Parasites**, used to detect parasitic infections like **Giardia** or **Cryptosporidium**, which can cause diarrhea.
- However, the absence of recent travel, city water, and the prominent greasy nature of the stool make this less likely as the primary initial investigation compared to confirming malabsorption.
*Stool guaiac test*
- This test detects **occult blood in stool**.
- The patient denies **melena or hematochezia**, and there are no signs pointing to gastrointestinal bleeding, making this test irrelevant for her presenting symptoms.
*CT of the abdomen with oral contrast*
- A CT scan with contrast might be used to investigate structural abnormalities or inflammation if other tests confirm malabsorption or point to a specific organ pathology (e.g., pancreatitis, Crohn's disease).
- It's an imaging study and generally not the most appropriate *initial* test for evaluating the described symptoms of malabsorption.
Laboratory diagnosis of parasites US Medical PG Question 2: A 31-year-old male traveler in Thailand experiences fever, headache, and excessive sweating every 48 hours. Peripheral blood smear shows trophozoites and schizonts indicative of Plasmodia infection. The patient is given chloroquine and primaquine. Primaquine targets which of the following Plasmodia forms:
- A. Schizont
- B. Hypnozoite (Correct Answer)
- C. Trophozoite
- D. Merozoite
- E. Sporozoite
Laboratory diagnosis of parasites Explanation: ***Hypnozoite***
- **Primaquine** is a **radical cure** for malaria caused by *Plasmodium vivax* and *Plasmodium ovale* because it targets the dormant **hypnozoite** forms in the liver.
- The presence of **hypnozoites** leads to relapses, as they can reactivate and re-initiate the erythrocytic cycle.
*Schizont*
- **Schizonts** are merozoite-producing forms in red blood cells (**erythrocytic schizonts**) or liver cells (**hepatic schizonts**).
- While chloroquine targets **erythrocytic schizonts**, primaquine's primary unique action is against the dormant liver stages.
*Trophozoite*
- **Trophozoites** are the feeding and growing stages of the parasite within red blood cells, which mature into schizonts.
- **Chloroquine** is highly effective against **erythrocytic trophozoites** and schizonts, resolving acute malarial symptoms.
*Merozoite*
- **Merozoites** are released from ruptured schizonts and infect new red blood cells during the erythrocytic cycle.
- No specific antimalarial drug solely targets **merozoites** as a primary form; they are an infective stage for red blood cells.
*Sporozoite*
- **Sporozoites** are the forms injected by infected mosquitoes, which then travel to the liver and infect hepatocytes.
- While some drugs like atovaquone have activity against sporozoites, primaquine is specifically indicated for destroying the **hypnozoite** stage, preventing relapses.
Laboratory diagnosis of parasites US Medical PG Question 3: A 68-year-old man presents to his physician for symptoms of chronic weight loss, abdominal bloating, and loose stools. He notes that he has also been bothered by a chronic cough. The patient’s laboratory work-up includes a WBC differential, which is remarkable for an eosinophil count of 9%. Stool samples are obtained, with ova and parasite examination revealing roundworm larvae in the stool and no eggs. Which of the following parasitic worms is the cause of this patient’s condition?
- A. Taenia saginata
- B. Taenia solium
- C. Strongyloides stercoralis (Correct Answer)
- D. Necator americanus
- E. Ascaris lumbricoides
Laboratory diagnosis of parasites Explanation: ***Strongyloides stercoralis***
- The presence of **larvae (rhabditiform)** in the stool, **pulmonary symptoms** (chronic cough), **gastrointestinal symptoms** (weight loss, bloating, loose stools), and **eosinophilia** are classic findings for *Strongyloides stercoralis* infection.
- Unlike most other intestinal nematodes, *Strongyloides* can establish an **autoinfection cycle**, meaning larvae can reinfect the host, leading to persistent and potentially severe infections even in immunocompetent individuals, without the need for external re-exposure or eggs in stool.
*Taenia saginata*
- This is a **tapeworm (cestode)** that causes taeniasis and is acquired by consuming undercooked beef.
- Diagnosis is typically made by finding **proglottids** or **eggs** in the stool, not larvae, and pulmonary symptoms are not characteristic.
*Taenia solium*
- This is another **tapeworm (cestode)**, acquired by consuming undercooked pork; it can cause taeniasis (intestinal infection) and cysticercosis (tissue infection).
- Similar to *T. saginata*, diagnosis involves finding **proglottids** or **eggs** in stool for intestinal infection, and it does not typically present with lung involvement or larvae in stool.
*Necator americanus*
- This is a **hookworm** that causes iron-deficiency anemia due to blood loss in the intestines.
- While it can cause some pulmonary symptoms as larvae migrate through the lungs, and gastrointestinal symptoms, the diagnostic hallmark is finding **eggs** in the stool, not larvae.
*Ascaris lumbricoides*
- This is the **giant roundworm**; infections are common and often asymptomatic, but heavy worm burdens can cause intestinal obstruction or malnutrition.
- While **pulmonary symptoms (Loeffler's syndrome)** can occur during larval migration, and eosinophilia is present, the diagnosis is confirmed by finding characteristic **mammillated eggs** in the stool, not larvae.
Laboratory diagnosis of parasites US Medical PG Question 4: 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?
- A. Clue cells on saline smear (Correct Answer)
- B. Hyphae
- C. Motile flagellates
- D. Polymorphonuclear cells (PMNs) to epithelial cell ratio of 2:1
- E. Gram-negative diplococci
Laboratory diagnosis of parasites Explanation: ***Clue cells on saline smear***
- The symptoms of **fish-like vaginal odor**, **gray, thin, and malodorous discharge** are highly suggestive of **bacterial vaginosis (BV)**.
- **Clue cells** are **epithelial cells** covered in bacteria and are the hallmark diagnostic feature of BV on wet mount.
*Hyphae*
- **Hyphae** (or pseudohyphae) are characteristic of **candidiasis** (yeast infection).
- Candidiasis typically presents with **thick, white, cottage-cheese-like discharge** and **vaginal itching**, which are not described.
*Motile flagellates*
- **Motile flagellates** (specifically *Trichomonas vaginalis*) are characteristic of **trichomoniasis**.
- Trichomoniasis usually presents with **frothy, greenish-yellow discharge**, **cervical petechiae ("strawberry cervix")**, and **vulvar irritation**, which are absent here.
*Polymorphonuclear cells (PMNs) to epithelial cell ratio of 2:1*
- An elevated **PMN count** (especially a ratio like 2:1) is indicative of **vaginal inflammation** or **infection** such as cervicitis or trichomoniasis, but is typically **absent or low** in **bacterial vaginosis**.
- **Bacterial vaginosis** is characterized by a *decrease* in lactobacilli and an *overgrowth* of anaerobic bacteria, and often has **minimal host inflammatory response**.
*Gram-negative diplococci*
- **Gram-negative diplococci** are characteristic of **gonorrhea**, specifically *Neisseria gonorrhoeae*.
- Gonorrhea often presents with **purulent discharge**, **dysuria**, or can be **asymptomatic**, and is usually associated with **cervicitis**, which is not indicated by the painless cervical mobilization.
Laboratory diagnosis of parasites US Medical PG Question 5: 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?
- A. Nucleic acid amplification tests (NAATs) (Correct Answer)
- B. Urethral biopsy
- C. Tzanck smear
- D. Leukocyte esterase dipstick test
- E. Gram stain
Laboratory diagnosis of parasites Explanation: ***Nucleic acid amplification tests (NAATs)***
- NAATs are the **most sensitive and specific diagnostic tools** for detecting common sexually transmitted infections (STIs) like **gonorrhea** and **chlamydia**, which present with urethral discharge, dysuria, and itching.
- They can identify the **genetic material** of the causative organisms directly from urine samples or urethral swabs, making them highly effective even with low bacterial loads.
*Urethral biopsy*
- A urethral biopsy is an **invasive procedure** generally reserved for investigating conditions like **strictures, tumors, or chronic inflammatory diseases** when other diagnostic methods are inconclusive.
- It is not a primary diagnostic tool for acute urethritis suspected to be an STI, as it carries risks and is unnecessary given the availability of less invasive options.
*Tzanck smear*
- The Tzanck smear is primarily used for diagnosing **herpes simplex virus (HSV) infections** by looking for multinucleated giant cells and intranuclear inclusions.
- While HSV can cause genital lesions, it typically does not present as a primary symptom of mucopurulent urethral discharge and dysuria without visible vesicles or ulcers, making it less likely in this scenario.
*Leukocyte esterase dipstick test*
- A leukocyte esterase dipstick test detects the presence of **white blood cells** in urine, indicating inflammation or infection in the urinary tract.
- While it can suggest urethritis, it is **not specific for the causative agent** and merely indicates inflammation, requiring further specific testing to identify the pathogen.
*Gram stain*
- A Gram stain of urethral discharge can rapidly identify Gram-negative intracellular diplococci suggestive of **gonorrhea** (Neisseria gonorrhoeae).
- However, its sensitivity for gonorrhea is lower than NAATs, especially in asymptomatic cases or for detecting other common causes of urethritis like **Chlamydia trachomatis**, which are not visible on Gram stain.
Laboratory diagnosis of parasites US Medical PG Question 6: The World Health Organization suggests the use of a new rapid diagnostic test for the diagnosis of malaria in resource-limited settings. The new test has a sensitivity of 70% and a specificity of 90% compared to the gold standard test (blood smear). The validity of the new test is evaluated at a satellite health center by testing 200 patients with a positive blood smear and 150 patients with a negative blood smear. How many of the tested individuals are expected to have a false negative result?
- A. 60 (Correct Answer)
- B. 15
- C. 135
- D. 155
- E. 195
Laboratory diagnosis of parasites Explanation: ***Correct Option: 60***
- **False negatives** occur in individuals who have the disease but test negative. This is directly related to the test's **sensitivity**.
- Given a sensitivity of 70%, 30% of actual positive cases (100% - 70%) will be missed. With 200 patients having a positive blood smear (meaning they have malaria), 30% of 200 is 0.30 × 200 = **60**.
*Incorrect Option: 15*
- This number represents the expected number of **false positives** (150 patients without disease × 10% false positive rate = 15).
- However, the question asks for **false negatives**, not false positives.
*Incorrect Option: 135*
- This value represents the number of **true negatives** (150 patients without malaria × 90% specificity = 135).
- It does not represent false negative results.
*Incorrect Option: 155*
- This appears to be a distractor number that doesn't correspond to any standard diagnostic test calculation in this scenario.
- It does not represent false negatives or any meaningful combination of the given parameters.
*Incorrect Option: 195*
- This number might be derived from incorrectly applying formulas or miscalculating the relationship between sensitivity and false negatives.
- It does not represent the correct calculation for false negatives.
Laboratory diagnosis of parasites US Medical PG Question 7: A 55-year-old man presents to the physician with complaints of 5 days of watery diarrhea, fever, and bloating. He has not noticed any blood in his stool. He states that his diet has not changed recently, and his family has been spared from diarrhea symptoms despite eating the same foods that he has been cooking at home. He has no history of recent travel outside the United States. His only medication is high-dose omeprazole, which he has been taking daily for the past few months to alleviate his gastroesophageal reflux disease (GERD). Which of the following is the most appropriate initial test to work up this patient’s symptoms?
- A. Stool toxin assay (Correct Answer)
- B. Colonoscopy
- C. Fecal occult blood test
- D. Stool culture
- E. Stool ova and parasite
Laboratory diagnosis of parasites Explanation: ***Stool toxin assay***
- The patient's presentation of **watery diarrhea** and fever, especially with a history of **high-dose omeprazole use**, strongly suggests **Clostridioides difficile infection**.
- **Omeprazole** (a proton pump inhibitor) reduces stomach acid, which can disrupt the normal gut flora and increase susceptibility to *C. difficile*; a **stool toxin assay** is the most direct diagnostic test for this infection.
*Colonoscopy*
- While a colonoscopy can visualize pseudomembranes associated with severe *C. difficile* colitis, it is an **invasive procedure** and not the initial diagnostic test of choice for suspected infectious diarrhea.
- It is usually reserved for cases with atypical presentations, suspected complications, or when other diagnostic tests are inconclusive.
*Fecal occult blood test*
- The patient describes **watery diarrhea** and specifically states he has **not noticed any blood in his stool**, making a fecal occult blood test unlikely to be helpful in this acute setting.
- This test is primarily used for screening **colorectal cancer** or identifying chronic gastrointestinal bleeding.
*Stool culture*
- A stool culture primarily identifies bacterial pathogens like *Salmonella*, *Shigella*, or *Campylobacter*, which typically cause diarrheal illnesses that may include **bloody stools** or have specific epidemiological links (e.g., foodborne outbreaks).
- Given the history of **omeprazole use** and the absence of blood, *C. difficile* is more likely than these common bacterial enteritides, and a stool culture does not detect *C. difficile* itself.
*Stool ova and parasite*
- This test is used to detect **parasitic infections** (e.g., Giardia, Cryptosporidium), which can cause watery diarrhea and bloating.
- However, given the specific risk factor of **omeprazole use**, **Clostridioides difficile** infection is a more probable diagnosis, making the stool toxin assay the more appropriate initial test.
Laboratory diagnosis of parasites US Medical PG Question 8: 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?
- A. Urine antigen assay (Correct Answer)
- B. CT Chest
- C. Direct immunofluorescent antibody test
- D. Stool culture
- E. Polymerase chain reaction
Laboratory diagnosis of parasites Explanation: ***Urine antigen assay***
- This patient presents with **pneumonia symptoms** (low-grade fever, dry cough, dyspnea, bilateral infiltrates) along with **gastrointestinal symptoms** (watery diarrhea) and **hyponatremia**, after attending a hotel meeting with other sick attendees. These are classic features of **Legionnaires' disease**.
- A **urine antigen assay** is a rapid and highly specific test for **Legionella pneumophila serogroup 1**, which causes the majority of Legionnaires' disease cases.
*CT Chest*
- A CT scan of the chest would provide more detailed imaging of the lung infiltrates but is typically used to characterize findings once pneumonia is diagnosed or to rule out other lung pathologies, not as an initial diagnostic test for the specific pathogen.
- While it can reveal characteristic patterns, it doesn't identify the causative organism and is not the most appropriate *next step in diagnosis* for a presumed Legionella infection.
*Direct immunofluorescent antibody test*
- A **direct immunofluorescent antibody (DFA) test** is used to identify legionella in respiratory secretions. However, collecting a sufficiently good sputum sample can be difficult, especially with a **dry cough**.
- Its sensitivity is lower than urine antigen testing for serogroup 1 and requires a respiratory sample, making it less convenient for initial diagnosis.
*Stool culture*
- While the patient has diarrhea, a **stool culture** would primarily detect typical bacterial enteric pathogens (e.g., Salmonella, Shigella, Campylobacter) and would not identify **Legionella**.
- The diarrhea, in this context, is likely an extrapulmonary manifestation of Legionnaires' disease caused by Legionella, not a separate primary enteric infection.
*Polymerase chain reaction*
- **PCR testing** can detect Legionella DNA in respiratory samples, offering high sensitivity and specificity.
- However, it is generally less rapid and widely available than the urine antigen test for initial diagnosis of Legionella pneumophila serogroup 1, which is the most common cause of Legionnaires' disease.
Laboratory diagnosis of parasites US Medical PG Question 9: 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?
- A. Epithelial cells covered by numerous bacterial cells
- B. Chains of cocci
- C. Motile round or oval-shaped microorganisms (Correct Answer)
- D. Numerous rod-shaped bacteria
- E. Budding yeast cells and/or pseudohyphae
Laboratory diagnosis of parasites Explanation: ***Motile round or oval-shaped microorganisms***
- The symptoms of **frothy vaginal discharge**, **strawberry cervix** (punctate red maculae), and the patient's sexual history are classic for **Trichomonas vaginalis** infection.
- On **wet mount microscopy**, *Trichomonas vaginalis* appears as **motile, flagellated, pear-shaped protozoa** that are round or oval-shaped.
*Epithelial cells covered by numerous bacterial cells*
- This describes **clue cells**, which are characteristic of **bacterial vaginosis**.
- Bacterial vaginosis typically presents with a **fishy odor** and a thin, gray-white discharge, not frothy or associated with a strawberry cervix.
*Chains of cocci*
- While various cocci can be part of the vaginal flora or indicate infection, **chains of cocci** (e.g., *Streptococcus*) are not a primary diagnostic finding for the presented symptoms.
- This morphology is not characteristic of common causes of **vaginitis** like trichomoniasis, candidiasis, or bacterial vaginosis.
*Budding yeast cells and/or pseudohyphae*
- These findings are indicative of a **candidal vulvovaginitis (yeast infection)**.
- Candidiasis typically presents with a thick, **curd-like vaginal discharge**, severe itching, and redness, which differs from the frothy discharge and strawberry cervix described.
*Numerous rod-shaped bacteria*
- While rod-shaped bacteria (e.g., lactobacilli) are a normal part of the vaginal flora, a significant increase in specific types of rod-shaped bacteria, like **Gardnerella vaginalis**, in the absence of lactobacilli can indicate **bacterial vaginosis**.
- However, this finding alone does not uniquely describe the key clinical features of **frothy discharge** and **strawberry cervix** seen in this patient.
Laboratory diagnosis of parasites US Medical PG Question 10: 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?
- A. CT scan of the abdomen
- B. IgG against deamidated gliadin peptide
- C. Schilling test
- D. Enteroscopy
- E. PAS-stained biopsy of small bowel (Correct Answer)
Laboratory diagnosis of parasites Explanation: ***PAS-stained biopsy of small bowel***
- The patient's history of travel to Bangladesh, chronic diarrhea, malabsorption (weight loss, fatigue, elevated fecal fat, macrocytic anemia), and negative celiac serology (anti-tissue transglutaminase IgA) are highly suggestive of **Whipple's disease**.
- **Periodic Acid-Schiff (PAS) staining** of a small bowel biopsy is the gold standard for diagnosing Whipple's disease, revealing **PAS-positive macrophages** containing *Tropheryma whipplei*.
*CT scan of the abdomen*
- While a CT scan can identify structural abnormalities or masses, it is not the most direct diagnostic test for **malabsorptive conditions** like Whipple's disease.
- It would likely show non-specific findings such as **bowel wall thickening** or **lymphadenopathy**, but not the definitive histological changes.
*IgG against deamidated gliadin peptide*
- This test is used to diagnose **celiac disease**, but the patient's IgA anti-tissue transglutaminase was already negative, and this IgG test is typically performed when IgA deficiency is suspected or in young children.
- Given the strong suspicion of an infectious etiology due to travel history and systemic symptoms, focusing solely on celiac serology is less appropriate as the first next step.
*Schilling test*
- The Schilling test is an **obsolete test** that was historically used to assess **vitamin B12 absorption** and differentiate causes of B12 deficiency (pernicious anemia, bacterial overgrowth, or pancreatic insufficiency).
- This test is **no longer performed in clinical practice** due to unavailability of radioactive B12; modern evaluation uses serum B12, methylmalonic acid, and homocysteine levels.
- While the patient has macrocytic anemia, the test would not directly address the underlying cause of fat malabsorption and systemic symptoms present.
*Enteroscopy*
- Enteroscopy allows for visualization and biopsy of the small bowel beyond the reach of a standard upper endoscopy.
- While useful for obtaining biopsies, simply performing an enteroscopy without knowing what to look for or what specific stain to request (referring to PAS) on the biopsy would be less targeted than ordering a **PAS-stained biopsy** specifically.
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