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USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A 25-year-old man is brought to the emergency department with severe burns on his body sustained in a house fire about an hour ago. His left lower leg was trapped under a burning beam, and as a result, he could not immediately escape from the house. Temperature is 38.5°C (101.3°F), pulse is 120/min, respirations are 20/min, and blood pressure is 105/55 mmHg. He is conscious but is moaning in pain. There is no singed hair on his face or nostrils, and he is breathing spontaneously. Physical examination shows 2nd-degree burns to the anterior chest, abdomen, and entire left leg. Which of the following most likely represents the total percentage of his whole body surface area involved in the burn according to the rule of nines?  

External References

First Aid









acute gastric ulcer p. 729

child abuse sign p. 579

classification p. 496

inhalational injuries and p. 701

shock with p. 321

sunburn p. 496

testosterone/methyltestosterone for p. 681

External Links


A burn is the damage that happens after something really hot like a fire, hot water or steam, or even a hot object comes into contact with skin.

But burn injuries can also be caused by extreme cold; electricity; some chemicals, like strong acids; or radiation, like from the sun or medical treatments.

Ultimately burns cause damage and inflammation of the skin.

The skin plays an important role in protecting underlying muscles, bones, ligaments and internal organs; forming a barrier to infectious pathogens; and preventing water loss from the body.

Now, the skin is divided into three layers--the epidermis, dermis, and hypodermis.

The epidermis forms the thin outermost layer of skin, and it’s made up of several layers of keratinocytes - which make and secrete glycolipids, which help to prevent water from easily seeping into and out of the body.

Underneath the epidermis is the thicker dermis layer that contains the nerves and blood vessels.

But the dermis is divided into two layers - a thin papillary layer just below the epidermis, and a deeper reticular layer.

The papillary layer contains fibroblasts which produce a connective tissue protein called collagen.

The fibroblasts are arranged in finger-like projections called papillae; each of which contains blood vessels and nerve endings.

Nerve endings found in this layer sense pain and fine touch, which allows you to feel something like a feather touching your arm.

The reticular layer of the dermis is even thicker than the papillary layer.

The collagen in the reticular layer is packed very tightly together, making it excellent tissue support.

In addition, fibroblasts in the reticular layer secrete elastin--which is a stretchy protein that gives skin its flexibility.

The reticular layer also contains the skin’s accessory structures like oil and sweat glands, hair follicles, lymphatic vessels, and nerves - and all of the blood vessels that serve these tissues. A type of nerve ending found here detects pressure or vibration, which allows you to feel someone grabbing your arm.


  1. "Robbins Basic Pathology" Elsevier (2017)
  2. "Harrison's Principles of Internal Medicine, Twentieth Edition (Vol.1 & Vol.2)" McGraw-Hill Education / Medical (2018)
  3. "Pathophysiology of Disease: An Introduction to Clinical Medicine 8E" McGraw-Hill Education / Medical (2018)
  4. "CURRENT Medical Diagnosis and Treatment 2020" McGraw-Hill Education / Medical (2019)
  5. "The global burden of injury: incidence, mortality, disability-adjusted life years and time trends from the Global Burden of Disease study 2013" Injury Prevention (2015)
  6. "Burns: an update on current pharmacotherapy" Expert Opinion on Pharmacotherapy (2012)
  7. "Initial management of a major burn: II—assessment and resuscitation" BMJ (2004)

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