WBR0762

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
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 were 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 most likely has the most myoglobin concentration?]]
Answer A	AnswerA::Biceps brachii muscle
Answer A Explanation	AnswerAExp::Biceps brachii muscle predominantly has type II muscle fibers.
Answer B	AnswerB::Quadriceps muscle
Answer B Explanation	AnswerBExp::Quadriceps muscle predominantly has type II muscle fibers.
Answer C	AnswerC::Vastus lateralis muscle
Answer C Explanation	AnswerCExp::Vastus lateralis muscle predominantly has type II muscle fibers.
Answer D	AnswerD::Soleus muscle
Answer D Explanation	AnswerDExp::Soleus muscle predominantly has type I muscle fibers.
Answer E	AnswerE::Triceps muscle
Answer E Explanation	AnswerEExp::Triceps muscle predominantly has type II muscle fibers.
Right Answer	RightAnswer::
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. Hence they are called "red" fibers (due to increased myoglobin concentrations). They are slow-twitch muscles that may sustained contractions and undergo oxidative phosphorylation. They are generally useful in structural muscles. In contrast, type II muscle fibers contain much less mitochondrial content and much less concentrations of myoglobin. Hence they are called "white" fibers. They are fast-twitching fibers. They undergo anaerobic glycolysis and are hypertrophied during weight-tranining and exercise. However, it is important to note that muscle fibers from different muscles within the body are not exclusively composed of just 1 type. In fact, 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 muscle is a a type I predominant muscle. On the other hand, biceps brachii, psoas, vastus lateralis, and triceps muscles are all type II predominant muscles. Of interest also, the intermediate vastus (in contrast to the lateral vastus) is type I predominant muscle. Educational Objective: Type I muscle fibers, such as the soleus muscle, contain less myoglobin concentrations than type II muscle fibers. Reference: Bekedam MA, van Beek-Harmsen BJ, van Mechelen W, et al. Myoglobin concentration in skeletal muscle fibers of chronic heart failure patients. J App Physiol (1985). 2009; 107(4):1138-43. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::triceps, WBRKeyword::biceps, WBRKeyword::brachii, WBRKeyword::soleus, WBRKeyword::muscle, WBRKeyword::muscles, WBRKeyword::vastus, WBRKeyword::lateralis, WBRKeyword::lateral, WBRKeyword::intermedius, WBRKeyword::intermediate, WBRKeyword::type I, WBRKeyword::type II, WBRKeyword::type 1, WBRKeyword::type 2, WBRKeyword::fiber, WBRKeyword::fibers, WBRKeyword::red, WBRKeyword::white, WBRKeyword::slow, WBRKeyword::fast, WBRKeyword::twitch, WBRKeyword::twitching, WBRKeyword::myoglobin, WBRKeyword::concentration, WBRKeyword::content, WBRKeyword::mitochondrial, WBRKeyword::mitochondria
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Order in Linked Questions	LinkedOrder::