1. B According to the passage, diffuse esophageal spasm is linked to malfunction of endogenous nitric oxide (NO) production. Since glycerin trinitrate successfully improves peristalsis and reduces chest pain, it is likely that it forms NO in the body (choice B is correct). In fact, it is a potent vasodilator and serves as a treatment for both DES and heart disease. Glyceryl trinitrate is another name for glycerin trinitrate (choice A is wrong), NO2 is nitrous oxide (“laughing gas,” choice C is wrong), and NO3− is nitrate (choice D is wrong).
2. C As described in the passage, calmodulin binds calcium and results in the activation of myosin light-chain kinase and subsequent phosphorylation of the myosin light-light-chain. Phosphorylation allows for cross bridging and smooth muscle contraction. By limiting the amount of calmodulin present, smooth muscle contraction in the digestive tract cannot take place as readily, and peristalsis cannot occur regularly. This leads to discomfort when trying to swallow food (choice C is correct). Diarrhea would be expected in a situation of increased digestive tract motility (choice A is wrong). With a deficiency of calmodulin, de-creased myosin light-chain phosphorylation would be expected (choice B is wrong), and thus dede-creased subsequent cross-bridge formation (choice D is wrong).
3. C An agonist produces or increases the physiological effect of interest. From the figure, SP activates smooth muscle contraction, thus its agonist would also do so (choice C is correct). Acetylcholinesterase removes ACh from the synaptic cleft, so increasing acetylcholinesterase would decrease stimulation to the smooth muscle cell (choice A is wrong). Destruction of the rostral portion of the dorsal motor nucleus would damage the vagus nerve and would prevent smooth muscle contraction (among a list of other, far more severe problems;
choice B is wrong). According to the figure, VIP inhibits smooth muscle contraction (choice D is wrong).
4. C From the passage, we know that esophageal peristalsis contractions last for 8-10 seconds, but drinking water occurs more rapidly than that and thus this would obstruct the next bolus of water from passing down the esophagus. By causing inhibition along the esophagus during repeated swallowing, it allows fluids to flow down the esophagus without interference from peristalsis (choice C is correct). Deglutitive inhibition will not increase the total quantity of the food that can be eaten in one sitting (although it may affect rate), nor will it increase storage capacity of the digestive tract as food is not stored in the esophagus (choices A and D are wrong). Deglutitive inhibition may prevent damage associated with food being forced down a contracted esophagus, but this would not be related to overuse (choice B is wrong).
5. D Given the medical student’s initial inability to completely swallow the roll, primary peristaltic waves must have failed to move the roll effectively down the esophagus (choices A and B can be eliminated). Thus, secondary peristaltic waves are needed to drive the food down the esophagus and (ideally) into the stom-ach. Increased cardiac sphincter tone (regulating entrance of food from the esophagus into the stomach) would initially prevent food from entering the stomach and lead to discomfort with each secondary peri-staltic wave (choice D is correct). The pyloric sphincter is at the end of the stomach and regulates entrance of chyme into the duodenum, thus increased tone at this sphincter is unlikely to cause the discomfort in this instance (choice C is wrong).
6. A While sympathetic stimulation generally results in a decrease in GI activity, esophageal peristalsis does not change with sympathetic activity; basic maintenance activities, like swallowing saliva, need to be able to continue, even if eating and drinking are not occurring as part of a fright-or-flight response (choice A is correct).
Drill
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THE MUSCULOSKELETAL SYSTEM AND SKIN
CHAPTER 15 PRACTICE PASSAGE
In 1736, John Freke of Saint Bartholomew’s Hospital described a disease in which large swellings of fused bone were felt on the back of a 14 year old patient. These swellings had been growing for three years and had formed a “fixed bony pair of bodice” on the patient’s torso. Until recently, the cause of fibrodysplasia ossificans progressiva (FOP) remained a mystery, but recent research has greatly expanded the understanding of the pathophysiology behind this disease.
FOP is characterized by two clinical features: malformations of the great toes and progressive heterotropic ossification (HO).
Extensive HO begins in the first decade of life and occurs with significant pain at sites of tissue damage and inflammation.
Heterotropic bone replaces skeletal muscle and connective tissue, but spares the diaphragm, tongue, and extraocular muscles. Attempts to surgically remove heterotropic bone stimulate more robust bone growth. HO progresses throughout the patient’s lifetime, often resulting in impaired mobility and life-threatening complications.
Recently, the gene responsible for FOP has been identified as ACVR1, which produces activin receptor protein IA; a missense
mutation in the activation domain of the receptor results in the disease. ACVR1 functions as a bone morphogenetic protein (BMP) receptor and remains constitutively activated upon mutation; this results in overactive osteogenesis.
Signal transduction inhibitors (STIs) for ACVR1 are an area of significant research in the treatment of FOP. The ATP-binding site on the BMP receptor can be exploited to ensure specificity of STIs in FOP treatment and avoid cross reacting with similar BMP receptors. Because STIs are not yet available, current treatment focuses on supportive therapy and managing inflammation.
1. A pathologist examines a biopsied tissue sample under high resolution and observes numerous red, mononucleated, striated cells. Is this an appropriate tissue sample for determining whether the patient is suffering from FOP?
A) Yes, because heterotropic bone replaces skeletal muscle.
B) Yes, because heterotropic bone replaces connective tissue.
C) No, because it is not a bone sample.
D) No, because FOP does not affect cardiac muscle.
2. Below is a pedigree of a family with a history of FOP.
Based on the occurrence of the disease in this family, what is the likely mode of inheritance for FOP?
A) Sex-linked dominant B) Autosomal dominant C) Sex-linked recessive D) Autosomal recessive
3. Which of the following is a potential molecular mechanism by which a patient could develop fibrodysplasia ossificans progressiva?
A) A mutation that generates a stop codon early in the coding region
B) A mutation in the promoter region of the gene
C) A mutation in the coding region that changes an amino acid
D) An insertion mutation in an intron
4. A patient with fibrodysplasia ossificans progressiva presents at the doctor’s office. Which of the following physical exam findings would you expect to find in this patient?
A) Tongue deviates to one side when the patient is instructed to stick it out
B) Nystagmus (rapid involuntary oscillations of the eye) C) Severe dyspnea (labored respiration)
D) Nuchal rigidity (neck stiffness)
Drill
MCAT COMPLETE5. According to the passage, which of the following is the most effective treatment for FOP?
A) Surgery removing the overgrown bone
B) Sonification of overgrown bone to aid in bone particle removal
C) Systemic glucocorticoids to reduce inflammation D) Oral calcitonin
6. In the development of an STI for the treatment of FOP, a researcher discovers an increase in muscular intracellular calcium in an animal model, which results in increased muscular tone. Which of the following is the LEAST likely cause?
A) The STI acts as a receptor antagonist on the motor endplate.
B) The STI inhibits acetylcholinesterase.
C) The STI decreases the sarcoplasmic reticulum membrane integrity.
D) The STI inhibits the Ca2+-ATPase of the sarcoplasmic reticulum.
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