WBR0762

Author	[[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Physiology
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A researcher is analyzing muscle biopsy sections from 10 volunteers. In his experiment, he biopsies 20 mg of muscles from different regions of the body using a special needle technique. Then, he types muscle fibers according to myofibrillar ATPase histochemistry and calculates the percentage of type I and type II fibers. Finally, vapor fixation and quantitative histochemical techniques are used and compared to blocks of myoglobin for calibration. The researcher concludes that myogbolin concentration varies significantly between different muscle samples. Biopsy of which of the following muscles has the most myoglobin concentration?]]
Answer A	AnswerA::Biceps brachii muscles
Answer A Explanation	AnswerAExp::The biceps brachii muscles are type II predominant muscles.
Answer B	AnswerB::Quadriceps muscles
Answer B Explanation	AnswerBExp::The quadriceps muscles are type II predominant muscles.
Answer C	AnswerC::Vastus lateralis muscle
Answer C Explanation	AnswerCExp::The vastus lateralis muscle is a type II predominant muscle.
Answer D	AnswerD::Soleus muscle
Answer D Explanation	AnswerDExp::The soleus muscle is a type I predominant muscle.
Answer E	AnswerE::Triceps muscles
Answer E Explanation	AnswerEExp::The triceps muscles are type II predominant muscles.
Right Answer	RightAnswer::D
Explanation	[[Explanation::The technique described in the experiment appropriately reflects the variation in fiber composition. Myoglobin concentration varies significantly based on type I vs. type II muscle fibers. Type I muscle fibers contain high mitochondrial and high myoglobin content. Accordingly, they are referred to as "red" fibers (due to increased myoglobin concentrations). They are slow-twitch muscles active in sustained contractions and undergo oxidative phosphorylation. They are generally present in structural muscles. In contrast, type II muscle fibers are fast-twitch fibers that contain fewer mitochondrial content and fewer concentrations of myoglobin. They are referred to as "white" fibers. They are fast-twitching fibers and undergo anaerobic glycolysis. They are hypertrophied during weight-training and exercise. Muscle fibers from different muscles within the body are not exclusively composed of just 1 type of muscle fibers. Muscles are generally composed of both muscle fiber types, but the percentage of each fiber type varies according to the utility of the muscle. The soleus and the intermediate vastus muscles are type I predominant muscle. On the other hand, biceps brachii, psoas, vastus lateralis, triceps, and vastus lateralis (lateral vastus) muscles are all type II predominant muscles. Educational Objective: Type I muscle fibers have less myoglobin concentrations than type II muscle fibers. The soleus and the intermediate vastus muscles are type I predominant muscle. On the other hand, biceps brachii, psoas, vastus lateralis, triceps, and lateral vastus muscles are all type II predominant muscles. References: Bekedam MA, van Beek-Harmsen BJ, van Mechelen W, et al. Myoglobin concentration in skeletal muscle fibers of chronic heart failure patients. J Appl Physiol (1985). 2009;107(4):1138–1143. First Aid 2014 page 417]]
Approved	Approved::Yes
Keyword	WBRKeyword::Soleus, WBRKeyword::Vastus lateralis, WBRKeyword::Lateral vastus, WBRKeyword::Muscles, WBRKeyword::Type 1 muscle fiber, WBRKeyword::Type 2 muscle fiber, WBRKeyword::Slow twitch, WBRKeyword::Fast twitch, WBRKeyword::Myoglobin, WBRKeyword::Mitochondria, WBRKeyword::Contraction, WBRKeyword::Hypertrophy, WBRKeyword::Biceps, WBRKeyword::Triceps, WBRKeyword::Quadriceps
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