A baby is born after the 32nd gestational week by cesarean delivery. The mother suffered from gestational diabetes; however, she had no other pregnancy-related diseases and was otherwise healthy. The baby has a blood pressure of 100/58 mm Hg, heart rate of 104/min, and oxygen saturation of 88%. The child has tachypnea, subcostal and intercostal retractions, nasal flaring, and cyanosis. The cyanosis is responding well to initial administration of oxygen. The nasogastric tube was positioned without problems. Which of the following is the most likely diagnosis?

Congenital heart anomaly with right-to-left shunt

In which of the following pathological states would the oxygen content of the trachea resemble the oxygen content in the affected alveoli?

Foreign body obstruction distal to the trachea

A 44-year-old man comes to the physician because of a 5-month history of persistent cough productive of thick, yellow sputum and worsening shortness of breath. One year ago, he had similar symptoms that lasted 4 months. He has smoked two packs of cigarettes daily for the past 20 years. Physical examination shows scattered expiratory wheezing and rhonchi throughout both lung fields. Microscopic examination of a lung biopsy specimen is most likely to show which of the following findings?

Increased ciliated epithelial cells

A 38-year-old man comes to the clinic complaining of recurrent abdominal pain for the past 2 months. He reports a gnawing, dull pain at the epigastric region that improves with oral ingestion. He has been taking calcium carbonate for the past few weeks; he claims that “it used to help a lot but it’s losing its effects now.” Laboratory testing demonstrated increased gastrin levels after the administration of secretin. A push endoscopy visualized several ulcers at the duodenum and proximal jejunum. What characteristics distinguish the jejunum from the duodenum?

Lack of submucosal Brunner glands

A 55-year-old man comes to the physician because of heartburn for the past 2 years. He has no chest pain, dysphagia, weight loss, or fever. He has no history of any serious illnesses. He takes omeprazole daily. Vital signs are within normal limits. Body mass index (BMI) is 34 kg/m2. Physical exam shows no abnormalities. An endoscopic image of the lower esophageal sphincter is shown. Which of the following is the most important next step in management?

Endoscopic mucosal ablation therapy

Laparoscopic Nissen fundoplication

A 26-year-old woman comes to the physician because she has not had a menstrual period for 5 weeks. Menarche was at the age of 14 years and menses occurred at regular 30-day intervals. She reports having unprotected sexual intercourse 3 weeks ago. A urine pregnancy test is positive. Which of the following best describes the stage of development of the embryo at this time?

Neural crest has formed, but limb buds have not yet formed

Fetal heart is beating, but cardiac activity is not yet visible on ultrasound

Limb buds have formed, but fetal movements have not begun

Sexual differentiation has begun, but fetal movement has not started

Implantation has occurred, but notochord has not yet formed

A 4-week-old boy is brought to the emergency department with a 2-day history of projectile vomiting after feeding. His parents state that he is their firstborn child and that he was born healthy. He developed normally for several weeks but started to eat less 1 week ago. Physical exam reveals a small, round mass in the right upper quadrant of the abdomen close to the midline. The infant throws up in the emergency department, and the vomitus is observed to be watery with no traces of bile. Which of the following is associated with the most likely cause of this patient's symptoms?

Nitric oxide synthase deficiency

Failure of neural crest migration

Respiratory system histology — USMLE Lesson

Conducting Zone - The Air Superhighway

Respiratory system histology: Trachea to alveoli

Function: Warms, humidifies, and filters inspired air. Lined predominantly by respiratory epithelium (ciliated pseudostratified columnar) with mucus-producing goblet cells.
Trachea & Bronchi:
- Supported by hyaline cartilage (C-rings in trachea, irregular plates in bronchi).
- Contain submucosal seromucous glands for mucus production.
Bronchioles:
- Lack cartilage and glands; this is a key identifier.
- Feature a prominent circumferential layer of smooth muscle (muscularis mucosae).
- Epithelium transitions from ciliated columnar to simple cuboidal.
Terminal Bronchioles:
- Final, smallest part of the conducting zone.
- Lined by simple cuboidal cells, including Club (Clara) cells which produce a surfactant component and act as stem cells.

⭐ Reid Index: The ratio of submucosal gland thickness to the bronchial wall thickness (measured from epithelium to cartilage). A normal index is < 0.4; it is characteristically increased in chronic bronchitis due to glandular hypertrophy.

Bronchioles & Club Cells - Tiny Tubes, Big Roles

Bronchioles: Airways <1 mm in diameter; transition from conducting to respiratory zones.
- Lack cartilage and submucosal glands.
- Lined by ciliated simple columnar to cuboidal epithelium.
- Prominent smooth muscle layer regulates airflow.
- Terminal bronchioles: Final segment of the conducting zone.
- Respiratory bronchioles: Have alveoli in their walls; begin the respiratory zone.
Club Cells (formerly Clara Cells): Non-ciliated, dome-shaped cells in terminal/respiratory bronchioles.
- Functions:
  - Secretion: Surfactant components & Club Cell Secretory Protein (CCSP/CC16).
  - Detoxification: Degrade inhaled toxins via cytochrome P450 enzymes.
  - Progenitor cells: Regenerate bronchiolar epithelium after injury.

⭐ Inhaled toxins can deplete Club cells, impairing detoxification and repair mechanisms in the small airways, a key pathological feature in diseases like COPD.

Histology of terminal bronchiole with Clara cells

Alveoli - The Gas Exchange Hub

Primary site of gas exchange; thin-walled, sac-like structures at the end of respiratory bronchioles.
Cellular Components:
- Type I Pneumocytes (95%): Simple squamous epithelium. Extremely thin for optimal gas diffusion. Form the alveolar side of the blood-air barrier.
- Type II Pneumocytes (5%): Cuboidal cells.
  - Synthesize & secrete pulmonary surfactant (dipalmitoylphosphatidylcholine), which ↓ surface tension.
  - Act as stem cells, regenerating both Type I and Type II cells.
- Alveolar Macrophages (Dust Cells): Phagocytose inhaled particles & pathogens. In heart failure, they become hemosiderin-laden "heart failure cells".
Pores of Kohn: Allow collateral ventilation between adjacent alveoli, preventing atelectasis.

⭐ Neonatal Respiratory Distress Syndrome (NRDS): Occurs in premature infants (< 35 weeks gestation) due to insufficient surfactant production. Leads to widespread alveolar collapse, requiring artificial surfactant and respiratory support.

Lung Alveolus Histology with Type I & II Pneumocytes

High‑Yield Points - ⚡ Biggest Takeaways

Respiratory epithelium transitions from pseudostratified ciliated columnar with goblet cells to simple cuboidal in terminal bronchioles.

Bronchioles are defined by a lack of cartilage and glands and the presence of Club cells (secretory, protective).

Type I pneumocytes are squamous cells covering 95% of the alveolar surface for gas exchange.

Type II pneumocytes are cuboidal, produce pulmonary surfactant, and act as stem cells for the alveolar lining.

Alveolar macrophages (dust cells) clear debris; they become hemosiderin-laden in heart failure.

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