A 44-year-old man with progressive dyspnea is diagnosed with dilated cardiomyopathy.  Despite optimal medical therapy, he continues to have symptoms and disease progression is noted.  He undergoes cardiac transplantation after a suitable donor becomes available.  Permission is obtained from the patient to study his diseased heart for intracellular calcium regulation.  Microelectrodes placed into cardiac muscle cells detect a rapid decrease in cytoplasmic calcium level immediately preceding relaxation.  Which of the following proteins is most likely responsible for the observed change in electrolyte levels?

A 65-year-old postmenopausal woman comes to the office for an annual visit.  The patient feels well and has no symptoms.  She has no chronic medical conditions, and her only medication is a daily multivitamin.  Physical examination shows no abnormalities.  Screening DXA scan reveals mild osteopenia.  The patient is advised on the appropriate lifestyle measures to limit bone loss, including weight-bearing exercise.  In response to increased mechanical loading, cells located within bone lacunae (indicated by the below arrow) increase bone remodeling within regions experiencing excessive mechanical strain.  These cells are most likely to directly perform which of the following functions during the remodeling process?

 researcher is investigating the structure of the sarcomere using skeletal muscle obtained from an experimental animal.  He develops monoclonal antibodies directed against a specific skeletal muscle protein and finds that these antibodies disrupt the binding of actin to structural support elements within the sarcomere.  Electron microscopy of the sarcomere is shown in the image below.

Which of the following labeled regions do these antibodies most likely bind?

Neurophysiologists are studying recordings of the membrane potential from a giant squid axon. A portion of their recordings is shown on the slide below.

The membrane is most permeable to potassium ions at which of the following points?

A researcher is investigating a new antiseizure drug that binds to and activates GABA-A receptors in the central nervous system.  During the experiment, she isolates a nerve and uses a microelectrode to measure its membrane potential under physiologic conditions.  The resting potential of the nerve is found to be −70 mV.  Which of the following membrane potentials will most likely be recorded from this neuron following exposure to the new drug?

Researchers are studying how the membrane potential of a postsynaptic neuron changes in response to neurotransmitter stimulation.  Baseline measurements determine that the resting membrane potential is generated by high membrane permeability for a particular ion.  When neurotransmitter stimulation begins, ligand-gated ion channels open (black arrow) and increase the membrane permeability for a different ion, causing a change in membrane potential.  This triggers the delayed opening of voltage-gated ion channels (red arrow), which increase the membrane permeability for a third type of ion.  The results of the experiment are shown in the graph below.

The equilibrium potentials of different ions under physiologic conditions are as follows:

Which of the following options would best explain the changes in this neuron’s membrane potential during the experiment?

A 38-year-old woman is being evaluated for intermittent tingling and numbness of the left hand for the past several months.  Her symptoms predominantly involve the fourth and fifth digits and are often brought on by driving a car.  Physical examination shows no abnormalities.  An electrodiagnostic study is performed, during which a nerve stimulator is placed on the left fifth digit and a surface recorder is placed over the ulnar nerve above the elbow.  Electrical stimulus is applied, and the graph obtained is shown below.  Results from the asymptomatic contralateral extremity are also shown.  Which of the following is primarily affected in this patient?

Researchers are studying treatment of osteoporotic fractures.  Subjects with pelvic fractures due to osteoporosis are divided into 2 groups.  The control group receives physical therapy along with calcium and vitamin D supplementation.  The experimental group receives a parathyroid hormone analogue in addition to the standard therapy.  It is found that the subjects who receive hormonal treatment have a shorter time to fracture healing and improved functional outcome.  The benefits of the investigational medication are attributed to increased osteoblastic activity.  An elevated level of which of the following markers is most likely to reflect the medication effect in these subjects?

The resting membrane potential for an isolated muscle cell is -70 mV.  Equilibrium potentials for important ions are given below:

E_Na = + 60 mV
E_K = – 80 mV
E_Cl = – 70 mV
E_Ca = + 125 mV

Which of the following most likely forms the resting membrane potential of the cell?

A 34-year-old man comes to the emergency department complaining of weakness, dizziness, and paresthesias involving his face and extremities.  He has never had symptoms like this before.  Two hours earlier, he ate fish at a new Japanese restaurant.  Neurological examination shows reduced muscle strength and decreased deep-tendon reflexes.  Which of the following parts of the action potential curve is most likely impaired in this patient?

A 67-year-old man comes to the office with right tibial pain that started 3 months ago and has increased in intensity over time.  He also has had progressive hearing impairment for the last year.  Physical examination reveals local tenderness and a lumpy protuberance over the right tibia.  After extensive evaluation, the patient undergoes a bone biopsy.  The pathologist identifies numerous multinucleated cells, some containing over 100 nuclei.  Which of the following factors is essential for the differentiation of the cells described by the pathologist?

A patient on comfort care in the hospital passes away due to natural causes.  The body is taken to the morgue for autopsy.  The pathologist begins an examination the next day and notes that the skeletal muscles across the patient’s body are significantly stiffened.  Failure of which of the following processes most strongly contributes to this phenomenon?

An 8-year-old boy is evaluated in the office due to difficulty walking.  The patient has problems running, jumping, and walking up steps.  There is no history of neurological disorders in the family.  Physical examination reveals weakness of lower extremity muscles bilaterally.  Electron microscopy of a biopsy sample taken from calf muscle is shown below.

Which of the following labeled regions contains only thick filaments and no thin filaments?

A muscle biopsy obtained from a healthy volunteer consists mainly of myoglobin-rich, glycogen-poor fibers with many mitochondria.  Which of the following is the most likely biopsy site?

Molecular biologists perform a series of experiments to characterize the electrophysiologic properties of human muscle cells.  The resting membrane potential for an isolated muscle cell is determined to be -70 mV.  The equilibrium potentials for important ions under normal physiologic conditions are as follows:

E_Na = + 60 mV
E_K = – 90 mV
E_Cl = – 75 mV
E_Ca = + 125 mV
E_Mg = 0 mV

If physiologic conditions are maintained, which of the following ions would most likely flow out of the cell after opening of their respective ion channels?

Physiologists are studying the biomolecular mechanisms underlying skeletal muscle contraction.  They have been analyzing muscle fibers obtained from knockout mice to determine how different cellular substances influence muscular contraction.  Striated muscle fibers obtained from a specific mouse embryo fail to contract in response to a substance normally released from the sarcoplasmic reticulum.  A deficiency of which of the following would best explain this finding?

An 18-year-old man, who was recently started on risperidone therapy for schizophrenia, comes to the office due to involuntary deviation of his head.  On examination, the patient’s head is tilted toward the right side and the chin is rotated to the left.  There is palpable tightening of the right sternocleidomastoid muscle.  It is determined that the patient is experiencing a medication-induced dystonic reaction due to motor neuron hyperactivity.  The persistent myocyte stimulation causes a substance to be released from the sarcoplasmic reticulum.  This substance most likely binds to which of the following proteins to cause this patient’s symptoms?

A 42-year-old weightlifter is preparing for an upcoming strength competition.  After lifting a series of progressively heavier weights, he decides to increase the weight on the exercise bar to 175 kg (385 lbs), the heaviest lift he has ever attempted.  He is able to hold the weight over his head for several seconds.  However, his arms suddenly and involuntarily give way and he drops the weight to the ground.  Which of the following structures was most likely responsible for the sudden muscle relaxation?

A 17-year-old boy is referred to a clinic for ongoing muscle weakness.  He has noticed difficulty climbing stairs at home over the last several months, and he recently stopped playing basketball due to difficulty lifting the ball over his head.  The patient takes no medications and does not know of any family members with muscle weakness.  On neurologic examination, he is unable to lift his arms or legs against resistance when fully outstretched.  He undergoes a muscle biopsy to evaluate the cause of his proximal muscle weakness.  Electron microscopy of the specimen reveals sparse transverse tubules in some of the muscle fibers.  Which of the following is the most likely consequence of this patient’s biopsy findings?

A group of investigators is studying the process of bone remodeling in response to steroid hormones.  Twenty adult female rats are randomly divided into 2 groups, with one group undergoing bilateral oophorectomy and the other undergoing sham laparotomy to serve as a control group.  Eight weeks after the surgery, bone samples are obtained from all animals.  Immunohistochemical evaluation shows overexpression of receptor activator of nuclear factor kappa B (RANK) on the surface of certain bone cells in the oophorectomized animals.  Which of the following is the most likely effect of the observed finding?