USMLE® Step 1 style questions USMLE
A 24-year-old man comes to the clinic because of a 2-week history of nonproductive cough and intermittent headaches. The patient has been feeling severely fatigued for the past few weeks, even though his daily regimen has remained the same. He is currently training to become an artillery officer and lives in army barracks. Past medical history is noncontributory, and he takes a multivitamin daily. He does not use tobacco, alcohol, or illicit substances. Temperature is 38.2°C (100.8°F), pulse is 96/min, and blood pressure is 128/85 mmHg. Physical examination reveals conjunctival pallor and pale mucous membranes. Bilateral crackles are heard on chest auscultation. Chest radiograph reveals diffuse bilateral infiltrates. Laboratory results are as follows:
|Complete blood count|
|Haptoglobin||23 mg/dL (N= 41-165 mg/dL)|
Further testing in this patient is likely to reveal which of the following findings?
Mycoplasma, as a genus, have a cell membrane that is packed with sterols, but they lack a proper, rigid cell wall.
Therefore, they don’t take up dye under Gram staining, so they can’t be visualized with light microscopy.
Additionally, they are highly pleomorphic bacteria, meaning they have no fixed shape and size, and they’re also osmotically unstable in the external environment.
So, to survive, Mycoplasmas invade host cells and live intracellularly.
Now, Mycoplasma pneumoniae is a facultative anaerobe, meaning it can live without oxygen if it has to, but it grows better in an aerobic environment.
So it prefers places like lungs or respiratory airways, where there is an unlimited flow of oxygen.
As a result, some people may carry this bacteria in their nose or throat, and when they sneeze or cough, these organisms get out in the form of small respiratory droplets.
And when other people inhale these droplets, they may get infected, especially when they spend a lot of time together in close quarters.
So Mycoplasma pneumoniae infections occur mostly in children who go to school, young adults in college, or military recruits.
Following inhalation of the pathogen droplets, Mycoplasma pneumoniae attaches to an epithelial cell in the respiratory tract, using a specialized attachment organelle which has an adhesive protein complex, called ‘adhesion protein P1’ at its tip.
Adhesion protein P1 attaches to the host cell surface, like the respiratory epithelial cell, and holds on for dear life.
This makes it much harder for the mucociliary clearance mechanisms, which normally remove any foreign pathogen out of the respiratory tract, to clear the bacteria.
So Mycoplasma pneumoniae multiplies and damages the respiratory epithelial cells in the process.
When they reach the lungs, this starts a local inflammatory response, and lung tissue fills with white blood cells, proteins, fluid, and even red blood cells if a nearby capillary gets damaged in the process - leading to a local cytotoxic effect.
So Mycoplasma pneumoniae avoid the battlefield by sneaking inside lung cells, where they remain dormant or replicate intracellularly.
As a result, individuals infected by Mycoplasma pneumoniae are often asymptomatic or may have nonspecific symptoms like fatigue, sore throat, mild fever and dry hacking cough - all of which aren’t typical of bacterial pneumonia - hence the name atypical pneumonia.
Besides, the person may not feel very sick, as opposed to a person suffering from other bacterial pneumonia - where they’ll surely be bedridden and suffering from more severe symptoms like dyspnea, or shortness of breath, fever, chest pain, and a productive cough.
This is why, sometimes a case of atypical pneumonia is also referred to as walking pneumonia.
Mycoplasma pneumoniae can also cause encephalitis, especially following atypical pneumonia in children.