USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 66 year-old woman comes to the emergency department because of worsening confusion for the past day. Past medical history is significant for Parkinson disease and diabetes mellitus type 2. She is admitted for observation and returns to baseline mentation after 3 days, but on the third day she begins complaining of worsening shortness of breath and cough productive of white sputum. The cough is also associated with chest pain during deep inhalation. Associated symptoms include sinus congestion and headache. Her temperature is 38.3°C (101.3°F), pulse is 92/min, respirations are 22/min, blood pressure is 140/90 mm Hg, and oxygen saturation is 94% on 2L O2 via nasal cannula. Labs and imaging are pending. At this time, which of the following is the most likely diagnosis?
Pneumonia is an infection in the lung tissue caused by microbes, and the result is inflammation.
The inflammation brings water into the lung tissue, and that extra water can make it harder to breathe.
Now, now in addition to air, you’re constantly breathing in other stuff, like microbes. But we’re usually good at protecting ourselves.
For example, we have mechanical techniques like coughing, a mucociliary escalator that lines the entire airway and moves out larger bacteria, and macrophages that are nestled deep inside the alveoli and ready to destroy anything that lands there.
But sometimes, a particularly nasty microbe might succeed in colonizing the bronchioles or alveoli, and when that happens - Congratulations! You’ve got pneumonia.
Those microbes typically multiply and cross over from the airways into the lung tissue, creating an inflammatory response.
The tissue quickly fills with white blood cells as well as proteins, fluid, and even red blood cells if a nearby capillary gets damaged in the process.
Now, there are lots of different pneumonia-causing microbes.
Usually it’s caused by viruses and bacteria, but it can also be caused by fungi and a special class of bacteria called mycobacteria.
There are also more unusual bacteria like mycoplasma pneumoniae, chlamydophila pneumoniae, and legionella pneumophila, which don’t have a cell wall and are well known for causing an “atypical or walking pneumonia” because they often cause vague symptoms like fatigue.
In individuals with a normal immune system, fungi are a rare cause of pneumonia and often it’s regional - for example, there’s Coccidioidomycosis in California and the Southwest - which you can remember because there’s a “C” in both cocci and california, Histoplasmosis in the Ohio and mississippi river valleys - “H” in Histo and in O”H”io, and Blastomycosis which are broad based budding yeast which are in the east - you can remember that with the “east” in yeast.
And the broad based budding refers to the fact that under a microscope, when the fungi bud off of each other there is a broad versus a narrow based.
To round out the fungal causes in the US, there’s Cryptococcus which is “cryptic” because geographically it can pop up really anywhere.
Now, one special fungal culprit is pneumocystis jiroveci which is a risk for immunocompromised individuals.
Finally, there’s mycobacteria which are slow-growing like fungi, hence the “myco” in their name even though they’re still bacteria.
The most well known one is mycobacterium tuberculosis, also just called TB.
Pneumonia can also be categorized by how it’s acquired.
The most common, is community acquired pneumonia, and it’s called that when a person gets sick outside of a hospital or healthcare setting.
This type tends to be more serious because sick patients often have a weakened immune systems and the microbes in hospitals are often resistant to the common antibiotics. That’s because hospitals bring together the bacteria that are often the most virulent - think great offense - as well as the most resistant - think great defence.
These bacteria are able to swap some of the antibiotic resistance genes with one another.
A well known example is Methicillin-resistant staphylococcus aureus, or MRSA.
MRSA on the other hand is resistant to many antibiotics and is therefore harder to treat.
Another category of pneumonia is ventilator associated pneumonia, which is a subset of the hospital-acquired pneumonia, but it specifically develops when ill individuals are connected to a ventilator.
Oftentimes, there’s a biofilm - which is a mix of bacteria and sugars and proteins that can coat a surface - that forms on the endotracheal tube.
Individuals on a ventilator can’t cough and are often quite sick already, so over time microbes can move from the tube directly into the lung and cause a pneumonia.
Now in addition to inhaling microbes there are other ways to develop pneumonia.