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First Aid USMLE STEP 1: Respiratory Questions with
Detailed Verified Answers (100% Correct Answers)
/Already Graded A+
How many periods does it take to develop the lung?
What are they?
Ans: 5
1) Embryonic (weeks 4-7)
2) Pseudoglandular (weeks 5-16)
3) Canalicular (weeks 16-26)
4) Saccular (weeks 26-birth)
5) Alveolar (weeks 32-8 years)
Describe the embryonic stage
Ans: Lung bud-> trachea-> mainstem bronchi-> secondary (lobar) bronchi-
> tertiary (segmental) bronchi
**Errors at this stage can lead to TE fistula
Describe the Pseudoglandular stage
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Ans: Endodermal tubules-> terminal bronchioles. Surrounded by modest
capillary network
**Respiration impossible, incompatible with life
Describe the Canalicular stage
Ans: Terminal bronchioles-> respiratory bronchioles-> alveolar ducts.
Surrounded by prominent capillary network
Describe the Saccular stage
Ans: Alveolar ducts-> terminal sacs.
Terminal sacs separated by primary septae. Pneumocytes develop.
Describe the Alveolar stage
Ans: Terminal sacs-> adult alveoli (d/t secondary septation).
In utero, “breathing” occurs via aspiration and expulsion of amniotic fluid->
increase in vascular resistance through gestation. At birth, fluid gets
replaced w/ air-> decrease in pulmonary vascular resistance
**At birth: 20-70 million alveoli
**By 8 years: 300-400 million alveoli
What are the 2 congenital lung malformations & describe them
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Ans: 1) Pulmonary hypoplasia= poorly developed bronchial tree w/
abnormal histology usually involving the right lung. Associated w/
congenital diaphragmatic hernia, bilateral renal agenesis (Potter Syndrome)
2) Bronchogenic cysts= Caused by abnormal budding of foregut & dilation
of terminal or large bronchi. Discrete, round, sharply defined & air-filled
densities on CXR. Drain poorly & cause chronic infections.
What are Type I pneumocytes?
Ans: thin squamous cells present in the alveoli, functioning in optimal gas
diffusion
Where are Type I pneumocytes found?
Ans: 97% of alveolar surfaces. (line the alveoli)
Role & epithelium of Type I pneumocytes
Ans: squamous. Thin for optimal gas diffusion
How is collapsing pressure calculated?
Ans: P = (2 x surface tension) / radius
What is the function of Type II pneumocytes?
Ans: secrete pulmonary surfactant —> decrease alveolar surface tension;
prevent alveolar collapse, decrease lung recoil & increase compliance
What type of cells, histologically, are Type II pneumocytes?
Ans: cuboidal
Do Type II cells originate from Type I cells, or are Type II cells progenitors
for Type I cells?
Ans: Type II cells are progenitors for Type I cells. Type II cells can also
give rise to other Type II cells.
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When do Type II cells proliferate?
Ans: in LUNG DAMAGE
What is the Law of Laplace?
Ans: As the radius decreases upon expiration, alveoli have an increased
tendency to collapse.
What does “atelectasis” mean, and how is it caused?
Ans: DEFINITION collapse of alveoli
CAUSES obstruction, compression, or contraction
—> damage to Type II pneumocytes —> loss of surfactant
NOTE Even reinflation may not return full function due to the loss of
surfactant.
What is surfactant, chemically?
Ans: a complex mix of lecithins, most importantly
DIPALMITOYLPHOSPHATIDYLCHOLINE
What are Clara (Club) cells?
Ans: nonciliated, columnar cells with secretory granules
What do Clara cells secrete?
Ans: a “watery” component of surfactant
What are the functions of Clara cells?
Ans: to secrete a component of surfactant, to degrade toxins, and to act as
reserve cells
When does surfactant synthesis begin?
Ans: around week 26 of gestation
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When are mature levels of surfactant reached?
Ans: around week 35 of gestation
If a child is born premature, is it likely that they will produce sufficient
levels of surfactant? If not, what is the child at risk of developing?
Ans: no
atelectasis
What measurement indicates if a fetus has mature lung function?
Ans: lecithin : sphingomyelin above 2
This can be measured in the amniotic fluid.
What is the cause of neonatal respiratory distress syndrome?
Ans: inadequate surfactant —> increased surface tension —> alveolar sac
collapse after expiration —> formation of hyaline membranes
What lecithin:sphingomyelin ratio in amniotic fluid is predictive of neonatal
RDS?
Ans: ratio <1.5
With what is neonatal RDS associated?
Ans: prematurity: adequate surfactant levels are not reached until week 35
C-section: d/t lack of release of stress-induced steroids (fetal
glucocorticoids) —> no increased synthesis of surfactant
maternal diabetes: increased fetal glucose-> increased fetal insulin->
decreased surfactant levels
What are the clinical features of neonatal RDS?
