A 25-year-old woman presents to an urgent care center following a bee sting while at a picnic with her friends. She immediately developed a skin rash and swelling over her arm and face. She endorses diffuse itching over her torso. Past medical history is significant for a mild allergy to pet dander and ragweed. She occasionally takes oral contraceptive pills and diphenhydramine for her allergies. Family history is noncontributory. Her blood pressure is 119/81 mm Hg, heart rate is 101/min, respiratory rate is 21/min, and temperature is 37°C (98.6°F). On physical examination, the patient has severe edema over her face and severe stridor with inspiration at the base of both lungs. Of the following options, this patient is likely experiencing which of the following hypersensitivity reactions?

Type 1 - anaphylactic hypersensitivity reaction

Type 2 - cytotoxic hypersensitivity reaction

Type 4 - cell mediated (delayed) hypersensitivity reaction

Type 3 - immune complex mediated hypersensitivity reaction

A 7-year-old male is admitted to the hospital with his fourth episode of wheezing and dyspnea. His symptoms are exacerbated by mold and pollen. Which of the following is most likely to be observed in this patient?

Ground glass opacities on chest CT

A 68-year-old male smoker dies suddenly in a car accident. He had smoked 2 packs per day for 40 years. His past medical history is notable for a frequent, very productive cough, recurrent respiratory infections and occasional wheezing. He had no other medical problems. At autopsy, which of the following is most likely to be found in this patient?

Increased number and activity of goblet cells

Interstitial fibrosis of the lung

As part of a clinical research study, microscopic analysis of tissues obtained from surgical specimens is performed. Some of these tissues have microscopic findings of an increase in the size of numerous cells within the tissue with an increase in the amount of cytoplasm, but the nuclei are uniform in size. Which of the following processes shows such microscopic findings?

Uterine myometrium in pregnancy

Liver following partial resection

Cervix with chronic inflammation

A 31-year-old man presents to his primary care physician with shortness of breath. He states that he had a “cold” 2 weeks ago and since then has had a persistent cough and worsening shortness of breath. He denies fever, chills, chest pain, sore throat, or rhinorrhea. His medical history is significant for seasonal allergies. He uses fluticasone nasal spray. He had his tonsils removed when he was 8 years of age. His mother and maternal grandfather have cirrhosis, and his father has depression and hypertension. The patient endorses that he smokes tobacco socially on the weekends and uses marijuana daily. He drinks 1-2 beers after work with his co-workers most evenings. A chest radiograph shows hyperinflation of the lungs and hyperlucency. Routine labs are drawn, as shown below. Serum: Na+: 139 mEq/L Cl-: 105 mEq/L K+: 4.0 mEq/L HCO3-: 26 mEq/L Urea nitrogen: 15 mg/dL Glucose: 100 mg/dL Creatinine: 0.8 mg/dL Alkaline phosphatase: 98 U/L Aspartate aminotransferase (AST, GOT): 46 U/L Alanine aminotransferase (ALT, GPT): 49 U/L Pulmonary function tests are pending. Which of the following is most likely to confirm the patient’s diagnosis?

Enzyme-linked immunosorbent assay

A 43-year-old man presents to the emergency department following a work-related accident in which both arms were amputated. The patient lost a substantial amount of blood prior to arrival, and his bleeding is difficult to control due to arterial damage and wound contamination with debris. His complete blood count (CBC) is significant for a hemoglobin (Hgb) level of 5.3 g/dL. The trauma surgery resident initiates the massive transfusion protocol and orders whole blood, O negative, which she explains is the universal donor. The patient receives 6 units of O negative blood prior to admission. He subsequently develops fever, chills, hematuria, and pulmonary edema. Several hours later, the patient goes into hemodynamic shock requiring the emergent administration of vasopressors. Of the following options, which hypersensitivity reaction occurred?

Type 2 hypersensitivity reaction

Type 1 hypersensitivity reaction

Combined type 1 and type 4 hypersensitivity reaction

Type 3 hypersensitivity reaction

Type 4 hypersensitivity reaction

Asthma pathology for USMLE Step 1: High-Yield Pathology Lessons

Asthma Basics - The Wheezy Setup

A chronic inflammatory disorder of the airways leading to reversible bronchoconstriction from bronchial hyperresponsiveness.
Clinically presents with episodic wheezing, cough, and shortness of breath.
Common Triggers:
- Allergens (e.g., pollen, dust mites)
- Viral respiratory infections
- Cold air, exercise
- Medications (Aspirin, β-blockers)

⭐ Atopic (extrinsic) asthma, the most common subtype, is a classic Type I Hypersensitivity reaction mediated by IgE, often with childhood onset.

Pathophysiology - Immune System Overdrive

Core Mechanism: A Type I Hypersensitivity reaction, driven by T-helper 2 (Th2) cells.
Initial Sensitization: Allergen exposure prompts APCs to activate Th2 cells, stimulating B-cells to produce IgE, which coats mast cells.

Phases of Attack:

Early Phase (Minutes): Re-exposure to allergen → IgE cross-linking on mast cells → degranulation. Releases histamine, leukotrienes (LTC₄, LTD₄, LTE₄) → potent bronchoconstriction, mucus production, and vasodilation.
Late Phase (Hours): Cytokines (esp. IL-5) from Th2 cells recruit eosinophils. Eosinophils release major basic protein and eosinophilic cationic protein, damaging epithelium and amplifying inflammation.

Asthma Pathophysiology: Atopic vs. Non-Atopic Mechanisms

⭐ Exam Favorite: Microscopic examination of sputum may reveal Curschmann spirals (whorled mucus plugs) and Charcot-Leyden crystals (crystalline aggregates of eosinophil proteins).

Gross & Microscopic - Airway Makeover Mess

Gross Findings
- Lungs are over-distended (hyperinflated) due to air trapping.
- Bronchi and bronchioles are occluded by thick, tenacious mucus plugs.
Microscopic Hallmarks (Airway Remodeling)
- Smooth muscle hypertrophy & hyperplasia (↑ muscle mass).
- Goblet cell hyperplasia in epithelium & submucosal gland hypertrophy (↑ mucus).
- Thickened basement membrane due to subepithelial fibrosis.
- Heavy inflammatory infiltrate, especially eosinophils and mast cells.

Asthma Bronchus Histology: Inflammation, Hypertrophy, Mucus

Key Luminal Findings
- Curschmann spirals: Spiral-shaped mucus plugs from shed epithelium.
- Charcot-Leyden crystals: Eosinophil-derived, needle-shaped crystals.

⭐ Airway remodeling can cause a progressive, irreversible loss of lung function over time, mimicking some features of COPD.

Clinical Correlation - Symptoms & Status

Classic Triad: Episodic wheezing, cough (esp. nocturnal), and dyspnea.
Pathophysiologic Link:
- Bronchoconstriction → Expiratory wheeze.
- Mucus hypersecretion/Plugging → Airflow obstruction, chest tightness.
- Inflammation & Edema → Airway hyperresponsiveness to triggers.
Status Asthmaticus:
- Severe, unremitting exacerbation; a medical emergency.
- Leads to respiratory muscle fatigue and potential failure.
- ⚠️ Ominous signs: "Silent chest" (no audible wheezing due to poor air entry), cyanosis, rising PaCO₂.

⭐ Pulsus Paradoxus: An exaggerated fall in systolic BP >10 mmHg during inspiration is a key indicator of severe asthma, reflecting negative intrathoracic pressure swings.

High‑Yield Points - ⚡ Biggest Takeaways

Reversible bronchoconstriction is the cardinal feature, a result of profound airway hyperresponsiveness to various stimuli.

Chronic inflammation is dominated by Th2 lymphocytes, eosinophils, and mast cell degranulation.

Key histologic findings include Curschmann spirals (whorled mucus plugs) and Charcot-Leyden crystals (from eosinophil protein).

Long-term airway remodeling features bronchial smooth muscle hypertrophy, sub-basement membrane fibrosis, and goblet cell hyperplasia.

Most cases represent a Type I hypersensitivity reaction initiated by common environmental allergens.

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