A 3-year-old girl is brought to the emergency department by her parents with sudden onset shortness of breath. They tell the emergency physician that their daughter was lying on the bed watching television when she suddenly began gasping for air. They observed a bowl of peanuts lying next to her when they grabbed her up and brought her to the emergency department. Her respirations are 25/min, the pulse is 100/min and the blood pressure is 90/65 mm Hg. The physical findings as of now are apparently normal. She is started on oxygen and is sent in for a chest X-ray. Based on her history and physical exam findings, the cause of her current symptoms would be seen on the X-ray at which of the following sites?

The posterior segment of the right lower lobe

The superior segment of the right lower lobe

The lingula of the left upper lobe

The apical segment of the right upper lobe

The apical segment of the left upper lobe

A 45-year-old male alcoholic presents with fever, productive cough, and foul-smelling sputum for the past two weeks. Vital signs are T 38.3 C, HR 106, BP 118/64 and RR 16. Oxygen saturation on room air is 90%. Given a diagnosis of aspiration pneumonia, initial chest radiograph findings would most likely include:

Right lung abscess due to the right main bronchus being wider and more vertically oriented

Mediastinal abscess located between vertebral levels T1-T3

Left lung abscess due to increased ventilation-perfusion ratio of the left lung

Right lung abscess due to increased anterior-posterior diameter of the right lung

Left lung abscess due to the left main bronchus being located superior to the right main bronchus

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 29-year-old mother brings in her 2-week-old baby boy to a pediatrician because he has been having difficulty feeding. The mother reveals that she had no prenatal care during her pregnancy and gave birth at home without complications. She says that her son seems to be having difficulty sucking, and she occasionally sees breast milk coming out of the infant’s nose. Physical exam reveals that this patient has a gap between his oral and nasal cavities behind the incisive foramen. He is therefore prescribed specialized bottles and his mom is taught positional techniques to ensure better feeding. Failure to fuse which of the following structures is most likely responsible for this patient's disorder?

Palatine shelves with nasal septum

Maxillary and medial nasal prominences

Nasal septum with primary plates

Maxillary and lateral nasal prominences

Palatine shelves with primary plates

A 35-year-old man is brought to the trauma bay by ambulance after sustaining a gunshot wound to the right arm. The patient is in excruciating pain and states that he can't move or feel his hand. The patient states that he has no other medical conditions. On exam, the patient's temperature is 98.4°F (36.9°C), blood pressure is 140/86 mmHg, pulse is 112/min, and respirations are 14/min. The patient is alert and his Glasgow coma scale is 15. On exam, he has a single wound on his right forearm without continued bleeding. The patient has preserved motor and sensation in his right elbow; however, he is unable to extend his wrist or extend his fingers further. He is able to clench his hand, but this is limited by pain. On sensory exam, the patient has no sensation to the first dorsal web space but has preserved sensation on most of the volar surface. Which of the following structures is most likely injured?

Recurrent motor branch of the median nerve

Respiratory system overview — USMLE Lesson

Upper Airways - The Entry Gates

Upper Respiratory System: Nasal Cavity, Pharynx, Larynx

Nose & Nasal Cavity: Filters, warms, and humidifies incoming air.
- Kiesselbach's Plexus: Anterior nasal septum; common site of epistaxis. 📌 LEGS mnemonic: Labial (Superior), Ethmoidal (Anterior), Greater Palatine, Sphenopalatine arteries.
- Turbinates (Conchae): Bony shelves that ↑ air turbulence and surface area.
Pharynx: Connects nasal cavity to larynx/esophagus.
- Nasopharynx: Respiratory epithelium; contains adenoids & Eustachian tube opening.
- Oropharynx & Laryngopharynx: Stratified squamous epithelium for food/air passage.
Larynx (Voice Box): Connects pharynx to trachea; for phonation and airway protection.

⭐ The cricoid cartilage is the narrowest part of a child's airway, while the vocal cords are the narrowest in adults. Cricothyrotomy is performed through the cricothyroid membrane.

Trachea & Bronchi - The Branching Pipes

Trachea and Bronchial Tree: Anterior View

Trachea: Extends from cricoid cartilage (C6) to carina (T4-T5). Supported by C-shaped hyaline cartilage rings.
Right Main Bronchus: Wider, shorter, and more vertical than the left.
- 📌 Mnemonic: "Inhale a bite, it goes down the right."
Left Main Bronchus: Passes inferior to the aortic arch and anterior to the esophagus.

⭐ Aspiration pneumonia most commonly affects the posterior segments of the right upper lobe and the superior segments of the right lower lobe in a supine patient due to the anatomy of the right main bronchus.

Lungs & Pleura - Spongy & Serious

Lungs: Spongy, air-filled organs responsible for gas exchange.
- Right Lung: 3 lobes (Superior, Middle, Inferior); separated by Oblique & Horizontal fissures.
- Left Lung: 2 lobes (Superior, Inferior) & a lingula (homolog of the middle lobe); separated by a single Oblique fissure.
- Hilum: "Root" of the lung where structures enter/exit.
  - Pulmonary artery, 2 Pulmonary veins, Main bronchus.
  - 📌 RALS: Right lung artery is Anterior; Left lung artery is Superior (relative to the main bronchus).
Pleura: A dual-layered serous membrane enveloping the lungs.
- Visceral Pleura: Adheres directly to the lung surface.
- Parietal Pleura: Lines the thoracic wall, diaphragm, and mediastinum.
- Pleural Cavity: Potential space between layers with serous fluid to reduce friction.

⭐ The costodiaphragmatic recess is the most dependent part of the pleural space, where fluid (pleural effusion) typically collects first in an upright individual.

Anatomy of the Lungs, Pleura, and Chest Wall

Innervation & Vasculature - The Support Crew

Pulmonary Blood Circulation and Bronchial Arteries

Innervation
- Autonomic: Pulmonary plexus
  - Parasympathetic: Vagus n. (CN X) → Bronchoconstriction, vasodilation, ↑ secretions.
  - Sympathetic: (T1-T4) → Bronchodilation, vasoconstriction, ↓ secretions.
- Somatic:
  - Phrenic n.: (C3, C4, C5) motor to diaphragm.
  - 📌 Mnemonic: "C3, 4, 5 keeps the diaphragm alive."
Vasculature
- Pulmonary arteries: Carry deoxygenated blood to lungs.
- Pulmonary veins (4): Carry oxygenated blood to the left atrium.
- Bronchial arteries: Supply nutrients/O₂ to lung tissue itself; arise from aorta.

⭐ Irritation of the phrenic nerve (e.g., by a tumor) can cause referred pain to the shoulder, as the C3-C5 spinal segments also provide sensory innervation to the shoulder area.

High‑Yield Points - ⚡ Biggest Takeaways

Aspiration is more common in the right main bronchus as it's wider, shorter, and more vertical.

The right lung has 3 lobes, while the left lung has 2 lobes and the lingula.

The diaphragm, the primary muscle of inspiration, is innervated by the phrenic nerve (C3, C4, C5).

Gas exchange occurs in the alveoli across the respiratory membrane.

The parietal pleura is sensitive to pain, while the visceral pleura is not.

The carina is the cartilage ridge at the bifurcation of the trachea.

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