USMLE® Step 1 Question of the Day: Oral loratadine

This week, we are sharing another USMLE® Step 1-style practice question to test your knowledge of medical topics. Today's case focuses on a 20-year-old woman with a history of well-controlled asthma presents with springtime allergies while training for a marathon. She experiences sneezing, rhinorrhea, and watery eyes. The physician prescribes loratadine. What is loratadine's mechanism of action? Test your diagnostic skills with this USMLE Step 1 question.

A 20-year-old woman presents to the primary care physician for routine follow-up in the springtime. The patient reports frequent sneezing, rhinorrhea and watery eyes since she started training for an upcoming marathon. Past medical history is also notable for well-controlled asthma. The patient uses an albuterol inhaler on an as-needed basis. The patient does not smoke, drink alcohol, or use recreational drugs. Temperature is 37.3°C (99.2°F ), pulse is 76/min, respirations are 16/min and blood pressure is 120/70 mmHg. Physical examination shows a prominent nasal crease. The nasal turbinates are boggy and bluish-gray. There is a copious thin and watery nasal mucus. The physician starts the patient on oral loratadine. The mechanism of this medication is best described as which of the following?

A. Antagonism of H1 receptors

B. Antagonism of H2 receptors

C. Stimulation of somatostatin receptors

D. Antagonism of nicotinic receptors

E. Antagonism of serotonin 5-HT3 receptors

Scroll down for the correct answer!

The correct answer to today's USMLE® Step 1 Question is...

A. Antagonism of H1 receptors

Before we get to the Main Explanation, let's look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

B. Antagonism of H2 receptors

Incorrect: H2 antagonists target the H2 receptors found in gastric parietal cells.  They are effective in the treatment of gastroesophageal reflux and peptic ulcers by blocking gastric acid secretion, but are not useful in the treatment of allergic rhinitis.

C. Stimulation of somatostatin receptors

Incorrect: Somatostatin receptors agonists (e.g. octreotide) inhibit bioactive amine release and are used to treat diarrhea in patients with carcinoid syndrome and VIPoma.

D. Antagonism of nicotinic receptors

Incorrect: Nicotinic receptor antagonists (e.g. rocuronium) are used to induce paralysis in preparation for surgery and endotracheal intubation. They are not effective in allergic rhinitis.

E. Antagonism of serotonin 5-HT3 receptors

Incorrect: Serotonin 5-HT3 receptor antagonists (e.g. ondansetron) are used primarily to treat nausea and vomiting. 

Main Explanation

This patient presents with sneezing, rhinorrhea and watery eyes consistent with allergic rhinitis.  Allergic rhinitis is an example of a Type I, or IgE mediated hypersensitivity reaction. Histamine is a small molecule stored in mast cells and basophils that mediates allergic, inflammatory and other physiologic processes throughout the body by binding to four different receptors (H1, H2, H3, H4).  Histamine plays an important role in Type I hypersensitivity reactions by binding to H1 receptors, causing local inflammation and vasodilation. 

In a Type 1 allergic reaction, an allergen, such as pollen, enters the body and activates B cells. Activated B-cells produce IgE antibodies that get released into the bloodstream and bind to mast cells. The mast cells are now “primed,” meaning that if pollen enters the body again in the future, the mast cells degranulate and release their histamine into the local tissue.

Antihistamines (e.g., diphenhydramine and loratadine) reduce itching, sneezing, and rhinorrhea by blocking H1 receptors on endothelial cells, smooth muscle cells, sensory nerve endings, and in the brain.

 

Major Takeaway

Type I hypersensitivity reactions are allergic reactions triggered by the binding of previously recognized antigen to IgE on mast cells. Histamine is one of the key chemical mediators released that causes local inflammation and vasodilation. Antihistamines (e.g. diphenhydramine and loratadine) reduce itching, sneezing, and rhinorrhea by blocking H1 receptors on endothelial cells, smooth muscle cells, sensory nerve endings and in the brain.

References

• Galli, S. J., Tsai, M., & Piliponsky, A. M. (2008). The development of allergic inflammation. Nature, 454(7203), 445-454.

• Leurs, R., Church, M. K., & Taglialatela, M. (2002). H1‐antihistamines: inverse agonism, anti‐inflammatory actions and cardiac effects. Clinical & Experimental Allergy, 32(4), 489-498.

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