USMLE-Rx Step 1 Practice Q's

USMLE-Rx Step 1 Qmax Challenge #1643

Check out today’s Step 1 Qmax Question Challenge.

Know the answer? Post it below! Don’t forget to check back for an update with the correct answer and explanation (we’ll post it in the comments section below).

A 25-year-old medical student presents to the clinic with a nonproductive cough, low-grade fever, and malaise of 3 weeks’ duration. A few friends in his study group have been complaining of similar symptoms. The patient denies exposure to farm animals, travel, or HIV.
In clinic, vital signs are:

Temperature: 37.2°C (99°F)
Heart rate: 80 beats/min
Respiratory rate: 18 breaths/min
Blood pressure: 110/75 mm Hg
Oxygen saturation: 96% on room air (normal range: 95%–99%)

On physical examination, lungs are clear to auscultation bilaterally without egophony or dullness to percussion. Sputum cultures are negative. The physician treats for an atypical pneumonia.

Which of the following is the most sensitive method for identifying the most likely causative pathogen?

A. Cold agglutinin test
B. Eosinophil cationic protein
C. Giemsa stain
D. Gram stain
E. Nasopharyngeal polymerase chain reaction

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10 replies »

    • E , since it is more sensitive than Cold agglutinin test . ELISA is less expensive and faster test with more sensitivity than Agglutinin test , but it was not in the option list.

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  1. The correct answer is E. Mycoplasma pneumoniae is the most common cause of atypical pneumonia. Though Mycoplasma pneumonia is largely a clinical diagnosis, multiplex polymerase chain reaction (PCR) from a nasopharyngeal sample can be used to detect Mycoplasma and is now the diagnostic test of choice for suspected Mycoplasma infections due to its high specificity and sensitivity. PCR remains the diagnostic test of choice for atypical pneumonia caused by Chlamydophila pneumoniae too. In patients who present with insidious onset of dry cough, low-grade fever, headache, myalgias, nausea, or emesis, an atypical pneumonia should be considered. Atypical pneumonias are mostly caused by Mycoplasma pneumoniae or viruses. Mycoplasma is a short rod, but it is difficult to culture and detect. It cannot be visualized on Gram stain due to a lack of a cell wall. It can be detected by the cold agglutinin testing, but this is not the preferred method of detection as it has low sensitivity and specificity, whereas PCR is much more reliable. X-ray of the chest is often more impressive than clinical exam findings and is characterized by a patchy interstitial infiltrate, like those shown here. Treatment consists of antibiotic therapy with a macrolide, usually azithromycin, for 5 days.

    A is not correct. Cold agglutinin tests is a nonspecific test for the identification of Mycoplasma pneumonia. A negative test does not exclude infection. The antibodies produced against Mycoplasma can act as autoantibodies against erythrocytes, eliciting a cold agglutination reaction.

    B is not correct. Eosinophil cationic protein (ECP) levels are found to be elevated in Mycoplasma pneumonia; however, these levels are nonspecific. Although elevated levels of ECP are seen in both Mycoplasma pneumonia and asthma, the increase in ECP is believed to be elevated due to damage to respiratory epithelium and hypersensitivity in the respiratory system. Polyermerase chain reaction is more accurate in diagnosis of Mycoplasma pneumonia.

    C is not correct. Microscopic exam is the gold standard for diagnosing malaria. It can also be used to identify other parasites too. The blood sample is most often stained with Giemsa, though Wright stain is commonly used in hospitals when Giemsa is unavailable, though it may make species identification more challenging. Expect malaria in a patient with symptoms of fevers, chills, malaise, headaches, nausea and vomiting, body aches, hepatosplenomegaly, and mild jaundice in an individual who recently traveled to a malaria-endemic area. In more severe presentations, malaria can cause neurologic abnormalities, severe anemia due to hemolysis, hemoglobinuria, acute respiratory distress syndrome, abnormalities in blood coagulation, low blood pressure from cardiovascular collapse, acute kidney failure, and metabolic acidosis.

    D is not correct. Gram stains are used to visualize gram-positive or gram-negative bacteria. Mycoplasma pneumoniae lacks a cell wall and therefore is not visible on Gram stain.

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