An 11-year-old boy presents with a 2-day history of uncontrollable shivering. During admission, the patient’s vital signs are within normal limits, except for a fluctuating body temperature registering as low as 35.0°C (95.0°F) and as high as 40.0°C (104.0°F), requiring alternating use of cooling and warming blankets. A complete blood count (CBC) is normal, and a chest radiograph is negative for consolidations and infiltrates. An MRI of the brain reveals a space-occupying lesion infiltrating the posterior hypothalamus and extending laterally. Which of the following additional findings are most likely, based on this patient’s physical examination?
Q82
A 25-year-old woman presents to her primary care physician for her yearly physical exam. She has no past medical history and says that she does not currently have any health concerns. On physical exam, she is found to have hyperactive patellar reflexes but says that she has had this finding since she was a child. She asks her physician why this might be the case. Her physician explains that there are certain cells that are responsible for detecting muscle stretch and responding to restore the length of the muscle. Which of the following is most likely a characteristic of these structures?
Q83
A 71-year-old man comes to the physician because of decreased sexual performance for the past 2 years. He reports that it takes longer for his penis to become erect, and he cannot maintain an erection for as long as before. His ejaculations have become less forceful. Once he has achieved an orgasm, he requires several hours before he can have another orgasm. He has been happily married for 40 years and he has no marital conflicts. His only medication is esomeprazole for gastroesophageal reflux disease. Examination shows coarse dark pubic and axillary hair. The skin of his lower extremity is warm to the touch; pedal pulses and sensation are intact. Rectal examination shows a symmetrically enlarged prostate with no masses. His fasting serum glucose is 96 mg/dL and his prostate-specific antigen is 3.9 ng/mL (N < 4). Which of the following etiologies is the most likely cause of the patient's symptoms?
Q84
A 56-year-old woman presents to the emergency department with an episode of nausea and severe unrelenting right upper abdominal pain. She had a cholecystectomy for gallstones a year earlier and has since experienced frequent recurrences of abdominal pain, most often after a meal. Her past medical history is otherwise unremarkable and she only takes medications for her pain when it becomes intolerable. Her physical exam is normal except for an intense abdominal pain upon deep palpation of her right upper quadrant. Her laboratory values are unremarkable with the exception of a mildly elevated alkaline phosphatase, amylase, and lipase. Her abdominal ultrasound shows a slightly enlarged common bile duct at 8 mm in diameter (N = up to 6 mm) and a normal pancreatic duct. The patient is referred to a gastroenterology service for an ERCP (endoscopic retrograde cholangiopancreatography) to stent her common bile duct. During the procedure the sphincter at the entrance to the duct is constricted. Which statement best describes the regulation of the function of the sphincter which is hampering the cannulation of the pancreatic duct in this patient?
Q85
A 34-year-old woman comes to the physician because she has not had her period for 4 months. Menses had previously occurred at regular 28-day intervals with moderate flow. A home pregnancy test was negative. She also reports recurrent headaches and has noticed that when she goes to the movies she cannot see the outer edges of the screen without turning her head to each side. This patient's symptoms are most likely caused by abnormal growth of which of the following?
Q86
A 60-year-old male engineer who complains of shortness of breath when walking a few blocks undergoes a cardiac stress test because of concern for coronary artery disease. During the test he asks his cardiologist about what variables are usually used to quantify the functioning of the heart. He learns that one of these variables is stroke volume. Which of the following scenarios would be most likely to lead to a decrease in stroke volume?
Q87
A 25-year-old man is admitted to the hospital after a severe motor vehicle accident as an unrestrained front-seat passenger. Appropriate life-saving measures are given, and the patient is now hemodynamically stable. Physical examination shows a complete loss of consciousness. There are no motor or ocular movements with painful stimuli. The patient has bilaterally intact pupillary light reflexes. The patient is placed in a 30° semi-recumbent position for further examination. What is the most likely finding on the examination of this patient's right ear?
Q88
A 38-year-old woman presents to the physician’s clinic with a 6-month history of generalized weakness that usually worsens as the day progresses. She also complains of the drooping of her eyelids and double vision that is worse in the evening. Physical examination reveals bilateral ptosis after a sustained upward gaze and loss of eye convergence which improves upon placing ice packs over the eyes and after the administration of edrophonium. Which of the following is an intrinsic property of the muscle group affected in this patient?
Q89
A 2-month-old girl with a previous diagnosis of DiGeorge syndrome is brought to the emergency department with her parents following a seizure. Her mother states that the baby had been inconsolable all day and refused to feed. She was born at 39 weeks gestation via spontaneous vaginal delivery. She is up to date on all vaccines. Upon arrival to the hospital her heart rate is 120/min, respiratory rate is 40/min, and temperature of 37.0°C (98.6°F). On examination, she is afebrile and somnolent and her fontanelles are open and soft. While attempting to take her blood pressure, the patient’s arm and hand flex sharply and do not relax until the cuff is released. A light tap on the cheek results in an atypical facial muscle twitch. A CMP and CBC are drawn and sent for analysis. Which of the following is the most likely cause?
Q90
A 17-year-old teenager presents to the clinic with her parents complaining of headaches and loss of vision which began insidiously 3 months ago. She describes her headaches as throbbing, mostly on her forehead, and severe enough to affect her daily activities. She has not experienced menarche. Past medical history is noncontributory. She takes no medication. Both of her parents are alive and well. Today, her blood pressure is 110/70 mm Hg, the heart rate is 90/min, the respiratory rate is 17/min, and the temperature is 37.0°C (98.6°F). Breasts and pubic hair development are in Tanner stage I. Blood work is collected and an MRI is performed (the result is shown). Decreased production of which of the following hormones is the most likely explanation for the patient's signs and symptoms?
Cardiovascular US Medical PG Practice Questions and MCQs
Question 81: An 11-year-old boy presents with a 2-day history of uncontrollable shivering. During admission, the patient’s vital signs are within normal limits, except for a fluctuating body temperature registering as low as 35.0°C (95.0°F) and as high as 40.0°C (104.0°F), requiring alternating use of cooling and warming blankets. A complete blood count (CBC) is normal, and a chest radiograph is negative for consolidations and infiltrates. An MRI of the brain reveals a space-occupying lesion infiltrating the posterior hypothalamus and extending laterally. Which of the following additional findings are most likely, based on this patient’s physical examination?
A. Polyuria
B. Hyperphagia
C. Galactorrhea
D. Sleep disturbances
E. Anorexia (Correct Answer)
Explanation: **Anorexia**
- **Hypothalamic lesions**, particularly those affecting the **lateral hypothalamus**, often lead to **anorexia** and **weight loss** due to the role of this region in stimulating appetite.
- The patient's presentation with **uncontrollable shivering** and **deregulated body temperature** further points to hypothalamic dysfunction, which can also disrupt feeding centers.
*Polyuria*
- **Polyuria** is typically associated with damage to the **posterior pituitary gland** or its connections to the hypothalamus, leading to **diabetes insipidus** (lack of ADH).
- While a hypothalamic lesion can affect ADH production, the primary symptom constellation in this case more strongly suggests disruption of other hypothalamic functions.
*Hyperphagia*
- **Hyperphagia** (increased appetite) and **obesity** are most commonly linked to damage to the **ventromedial hypothalamus**, which is considered the satiety center.
- The presented lesion is described as infiltrating the posterior hypothalamus and extending laterally, making hyperphagia less likely than anorexia.
*Galactorrhea*
- **Galactorrhea** is often caused by **hyperprolactinemia**, which can result from lesions compressing the **pituitary stalk** or interfering with **dopamine's inhibitory effect** on prolactin release in the anterior pituitary.
- While a large hypothalamic lesion could theoretically impact pituitary function secondarily, galactorrhea is not a direct or most likely consequence of a posterior and lateral hypothalamic lesion.
*Sleep disturbances*
- While the **hypothalamus** plays a critical role in **sleep-wake cycles** (e.g., the **suprachiasmatic nucleus**), **uncontrollable shivering** and **poikilothermia** point more directly to acute disruption of temperature regulation rather than sleep disturbances as the most prominent likely additional finding.
- Many types of brain lesions can cause sleep disturbances, but the specific presentation here suggests a more particular set of hypothalamic dysfunctions.
Question 82: A 25-year-old woman presents to her primary care physician for her yearly physical exam. She has no past medical history and says that she does not currently have any health concerns. On physical exam, she is found to have hyperactive patellar reflexes but says that she has had this finding since she was a child. She asks her physician why this might be the case. Her physician explains that there are certain cells that are responsible for detecting muscle stretch and responding to restore the length of the muscle. Which of the following is most likely a characteristic of these structures?
A. They inhibit the activity of alpha-motoneurons
B. They activate inhibitory interneurons
C. They are in parallel with extrafusal skeletal muscle fibers (Correct Answer)
D. They are in series with extrafusal skeletal muscle fibers
E. They are innervated by group Ib afferent neurons
Explanation: ***They are in parallel with extrafusal skeletal muscle fibers***
- The structures described are **muscle spindles**, which are **stretch receptors** located within the muscle belly and arranged in parallel with **extrafusal muscle fibers**.
- This parallel arrangement allows them to detect changes in **muscle length** and the rate of change of length, initiating the **stretch reflex**.
*They inhibit the activity of alpha-motoneurons*
- Muscle spindles, primarily through their **Ia afferent fibers**, **excite** alpha-motoneurons, leading to muscle contraction and opposing the stretch.
- **Inhibition** of alpha-motoneurons is typically associated with structures like **Golgi tendon organs**, which respond to muscle tension.
*They activate inhibitory interneurons*
- While muscle spindles do excite **excitatory interneurons** that activate synergistic muscles, their primary action in the monosynaptic stretch reflex is direct excitation of **alpha-motoneurons**.
- **Inhibitory interneurons** are typically involved in mediating **reciprocal inhibition** of antagonistic muscles in response to muscle spindle activation.
*They are in series with extrafusal skeletal muscle fibers*
- Structures arranged in **series** with extrafusal muscle fibers are **Golgi tendon organs**, which are tension receptors.
- Muscle spindles are oriented **in parallel** to detect changes in muscle length.
*They are innervated by group Ib afferent neurons*
- Muscle spindles are primarily innervated by **group Ia (primary) afferent neurons** and **group II (secondary) afferent neurons**, which detect muscle length and rate of change of length.
- **Group Ib afferent neurons** innervate **Golgi tendon organs**, which are sensitive to muscle tension.
Question 83: A 71-year-old man comes to the physician because of decreased sexual performance for the past 2 years. He reports that it takes longer for his penis to become erect, and he cannot maintain an erection for as long as before. His ejaculations have become less forceful. Once he has achieved an orgasm, he requires several hours before he can have another orgasm. He has been happily married for 40 years and he has no marital conflicts. His only medication is esomeprazole for gastroesophageal reflux disease. Examination shows coarse dark pubic and axillary hair. The skin of his lower extremity is warm to the touch; pedal pulses and sensation are intact. Rectal examination shows a symmetrically enlarged prostate with no masses. His fasting serum glucose is 96 mg/dL and his prostate-specific antigen is 3.9 ng/mL (N < 4). Which of the following etiologies is the most likely cause of the patient's symptoms?
A. Physiologic (Correct Answer)
B. Psychogenic
C. Neoplastic
D. Vascular
E. Neurogenic
Explanation: ***Physiologic***
- The patient's symptoms, including **longer erection time**, inability to **maintain an erection**, less forceful ejaculations, and a prolonged **refractory period**, are all common age-related changes in male sexual function.
- Absence of specific underlying medical conditions (normal glucose, intact sensation and pulses) and marital conflicts further supports a **physiologic aging process**.
*Psychogenic*
- This is less likely as the patient reports a **happy marriage** and no marital conflicts, ruling out a common psychogenic cause for sexual dysfunction.
- Psychogenic erectile dysfunction typically presents with sudden onset, situational dysfunction, and often preserves **nocturnal erections**, which are not mentioned here.
*Neoplastic*
- While an enlarged prostate is noted, the **PSA is within normal limits** for his age (< 4 ng/mL), and the examination reveals no masses, making prostate cancer an unlikely cause of the sexual dysfunction.
- Sexual dysfunction as a primary symptom of prostate cancer is rare unless advanced disease causes significant hormonal changes or nerve involvement.
*Vascular*
- The presence of **warm skin** to the touch and **intact pedal pulses** and sensation in the lower extremities makes significant vascular compromise an unlikely cause of erectile dysfunction.
- Vascular erectile dysfunction typically presents with a more sudden onset and may be associated with risk factors like **diabetes, hypertension, or hyperlipidemia**, which are not clearly evident here.
*Neurogenic*
- **Intact sensation** in the lower extremities makes a broad neurogenic cause for erectile dysfunction less likely.
- Neurogenic causes often present with other neurological deficits or are associated with conditions like **diabetes (neuropathy)**, **spinal cord injury**, or **multiple sclerosis**, which are not indicated.
Question 84: A 56-year-old woman presents to the emergency department with an episode of nausea and severe unrelenting right upper abdominal pain. She had a cholecystectomy for gallstones a year earlier and has since experienced frequent recurrences of abdominal pain, most often after a meal. Her past medical history is otherwise unremarkable and she only takes medications for her pain when it becomes intolerable. Her physical exam is normal except for an intense abdominal pain upon deep palpation of her right upper quadrant. Her laboratory values are unremarkable with the exception of a mildly elevated alkaline phosphatase, amylase, and lipase. Her abdominal ultrasound shows a slightly enlarged common bile duct at 8 mm in diameter (N = up to 6 mm) and a normal pancreatic duct. The patient is referred to a gastroenterology service for an ERCP (endoscopic retrograde cholangiopancreatography) to stent her common bile duct. During the procedure the sphincter at the entrance to the duct is constricted. Which statement best describes the regulation of the function of the sphincter which is hampering the cannulation of the pancreatic duct in this patient?
A. The sphincter is contracted between meals.
B. A hormone released by duodenal cells that stimulates gastrointestinal motility is the most effective cause of relaxation.
C. Regulation of function of the sphincter of Oddi does not involve neural inputs.
D. A hormone released by the I cells of the duodenum in the presence of fatty acids is the most effective cause of relaxation. (Correct Answer)
E. Sphincter relaxation is enhanced via stimulation of opioid receptors.
Explanation: ***A hormone released by the I cells of the duodenum in the presence of fatty acids is the most effective cause of relaxation.***
- The **sphincter of Oddi** relaxes primarily in response to **cholecystokinin (CCK)**, which is released by **I cells** in the duodenum when **fatty acids** and amino acids enter the small intestine. This relaxation allows bile and pancreatic enzymes to flow into the duodenum to aid digestion.
- The patient's symptoms of post-cholecystectomy pain and dilated common bile duct suggest **sphincter of Oddi dysfunction**, where the sphincter fails to relax properly, hindering bile flow.
*The sphincter is contracted between meals.*
- While the sphincter of Oddi does maintain a **tonic contraction** between meals to prevent bile reflux and store bile in the gallbladder, this statement doesn't describe the primary mechanism for its **relaxation during digestion**.
- The problem in this patient is the failure of the sphincter to relax, rather than its normal contracted state.
*A hormone released by duodenal cells that stimulates gastrointestinal motility is the most effective cause of relaxation.*
- This likely refers to **motilin**, which is released by enterochromaffin cells (Mo cells) in the duodenum and primarily regulates gastrointestinal motility, particularly the migrating motor complex.
- Motilin does not directly or most effectively cause relaxation of the sphincter of Oddi; **CCK** from I cells is the primary hormonal mechanism.
*Regulation of function of the sphincter of Oddi does not involve neural inputs.*
- The function of the sphincter of Oddi is under **complex neural and hormonal control**.
- Both **parasympathetic (vagal)** and **sympathetic** nervous inputs modulate its activity, often working in conjunction with CCK to coordinate bile and pancreatic juice release.
*Sphincter relaxation is enhanced via stimulation of opioid receptors.*
- **Opioid receptors** in the sphincter of Oddi, when stimulated, can actually cause **contraction** or spasm of the sphincter.
- This is a well-known side effect that can worsen or induce biliary colic, not enhance relaxation.
Question 85: A 34-year-old woman comes to the physician because she has not had her period for 4 months. Menses had previously occurred at regular 28-day intervals with moderate flow. A home pregnancy test was negative. She also reports recurrent headaches and has noticed that when she goes to the movies she cannot see the outer edges of the screen without turning her head to each side. This patient's symptoms are most likely caused by abnormal growth of which of the following?
A. Adenohypophysis (Correct Answer)
B. Astrocytes
C. Schwann cells
D. Pineal gland
E. Arachnoid cap cells
Explanation: ***Adenohypophysis***
- The patient's symptoms of **amenorrhea**, recurrent headaches, and **bitemporal hemianopsia** (cannot see the outer edges of the screen) strongly suggest a **pituitary adenoma**.
- Pituitary adenomas typically arise from the **adenohypophysis** and can cause hormonal imbalances (leading to amenorrhea) and compress the **optic chiasm** (leading to visual field defects).
*Astrocytes*
- **Astrocytes** are glial cells that support neurons and form the **blood-brain barrier**; their abnormal growth typically leads to **gliomas**, which present with different symptoms like seizures, focal neurological deficits, or increased intracranial pressure, rather than specific endocrine dysfunction and bitemporal hemianopsia.
- While gliomas can cause headaches and visual field defects, the combination with **amenorrhea** points away from this diagnosis.
*Schwann cells*
- **Schwann cells** produce myelin in the peripheral nervous system; abnormal growth usually results in **schwannomas**, which present with symptoms related to cranial nerve or spinal nerve compression (e.g., hearing loss in acoustic neuroma), not typically bitemporal hemianopsia or amenorrhea.
- Schwannomas are derived from the myelin sheaths of peripheral nerves and would not explain the endocrine disturbances seen here.
*Pineal gland*
- Lesions of the **pineal gland** often cause **Parinaud syndrome** (failure of upward gaze, pupillary abnormalities) due to compression of the superior colliculi, and can also lead to **hydrocephalus** or precocious puberty.
- While headaches can occur, **bitemporal hemianopsia** and **amenorrhea** are not characteristic presentations of pineal gland tumors.
*Arachnoid cap cells*
- Abnormal growth of **arachnoid cap cells** leads to **meningiomas**, which are typically slow-growing tumors arising from the meninges.
- Meningiomas can cause headaches and focal neurological deficits depending on their location, but they do **not typically cause bitemporal hemianopsia** or **endocrine dysfunction** like amenorrhea, as they are usually external to the brain tissue.
Question 86: A 60-year-old male engineer who complains of shortness of breath when walking a few blocks undergoes a cardiac stress test because of concern for coronary artery disease. During the test he asks his cardiologist about what variables are usually used to quantify the functioning of the heart. He learns that one of these variables is stroke volume. Which of the following scenarios would be most likely to lead to a decrease in stroke volume?
A. Anxiety
B. Heart failure (Correct Answer)
C. Exercise
D. Pregnancy
E. Digitalis
Explanation: ***Heart failure***
- In **heart failure**, the heart's pumping ability is impaired, leading to a reduced **ejection fraction** and thus a decreased **stroke volume**.
- The weakened myocardium cannot effectively contract to expel the normal volume of blood, resulting in lower blood output per beat.
*Anxiety*
- **Anxiety** typically causes an increase in **sympathetic nervous system** activity, leading to increased heart rate and myocardial contractility.
- This often results in a temporary **increase in stroke volume** due to enhanced cardiac performance, not a decrease.
*Exercise*
- During **exercise**, there is a significant **increase in venous return** and sympathetic stimulation, leading to increased **end-diastolic volume** and contractility.
- This physiological response causes a substantial **increase in stroke volume** to meet the body's higher oxygen demands.
*Pregnancy*
- **Pregnancy** leads to significant **physiological adaptations** to accommodate the growing fetus, including a substantial increase in **blood volume**.
- This increased blood volume and cardiac output result in an **increase in stroke volume** to maintain adequate perfusion for both mother and fetus.
*Digitalis*
- **Digitalis** is a cardiac glycoside that **increases intracellular calcium** in myocardial cells, enhancing the **force of contraction**.
- This positive inotropic effect leads to an **increased stroke volume** by improving the heart's pumping efficiency.
Question 87: A 25-year-old man is admitted to the hospital after a severe motor vehicle accident as an unrestrained front-seat passenger. Appropriate life-saving measures are given, and the patient is now hemodynamically stable. Physical examination shows a complete loss of consciousness. There are no motor or ocular movements with painful stimuli. The patient has bilaterally intact pupillary light reflexes. The patient is placed in a 30° semi-recumbent position for further examination. What is the most likely finding on the examination of this patient's right ear?
A. Cold water causing ipsilateral saccadic movement.
B. Warm water mimicking the head turning left.
C. Warm water causing contralateral slow tonic deviation. (Correct Answer)
D. Cold water causing contralateral slow tonic deviation.
E. No response to warm water irrigation.
Explanation: ***Warm water causing contralateral slow tonic deviation.***
- The patient has suffered **severe brain injury** with loss of consciousness and absent motor/ocular movements, but **intact pupillary reflexes** indicate the midbrain is functional and suggests an intact brainstem below the level of cortical injury.
- **Caloric testing (oculovestibular reflex)** evaluates brainstem function in comatose patients. In a comatose patient with intact brainstem pathways, only the **slow tonic phase** of eye deviation occurs (the fast corrective phase requires an intact cortex).
- **Warm water irrigation** in the right ear causes endolymph flow that stimulates the horizontal semicircular canal, mimicking head rotation toward the irrigated side. This triggers the vestibulo-ocular reflex, causing eyes to deviate in the **opposite direction** (contralateral) to maintain gaze stability.
- In this comatose patient, **warm water in the right ear** produces **slow tonic deviation to the left** (contralateral), without the fast corrective phase.
- Mnemonic: **COWS** - Cold Opposite, Warm Same (refers to fast phase in awake patients); in coma, only slow phase remains, so warm → contralateral deviation.
*Cold water causing ipsilateral saccadic movement.*
- **Saccadic movements** are rapid, voluntary eye movements that require an intact cortex. In a comatose patient, saccades are absent.
- Cold water would cause **ipsilateral tonic deviation** (not saccadic movement) in a comatose patient with intact brainstem.
*Warm water mimicking the head turning left.*
- This option is ambiguous and doesn't clearly describe the expected eye movement finding.
- While warm water does mimic head turning toward the irrigated side, the resulting eye movement is **contralateral deviation**, not simply "mimicking head turning left."
*Cold water causing contralateral slow tonic deviation.*
- **Cold water irrigation** in the right ear would cause eyes to deviate **ipsilaterally** (toward the right), not contralaterally.
- Cold water causes endolymph flow opposite to warm water, resulting in eye deviation toward the irrigated ear in comatose patients.
*No response to warm water irrigation.*
- **Absent caloric responses** bilaterally indicate severe brainstem damage or brain death.
- This patient has **intact pupillary light reflexes**, which demonstrates functional midbrain (cranial nerve III) and suggests preserved brainstem pathways for the vestibulo-ocular reflex.
- A complete lack of response would be inconsistent with intact pupillary reflexes.
Question 88: A 38-year-old woman presents to the physician’s clinic with a 6-month history of generalized weakness that usually worsens as the day progresses. She also complains of the drooping of her eyelids and double vision that is worse in the evening. Physical examination reveals bilateral ptosis after a sustained upward gaze and loss of eye convergence which improves upon placing ice packs over the eyes and after the administration of edrophonium. Which of the following is an intrinsic property of the muscle group affected in this patient?
A. A small mass per motor unit
B. High ATPase activity (Correct Answer)
C. High myoglobin content
D. High density of mitochondria
E. Increased amount of ATP generated per molecule of glucose
Explanation: ***High ATPase activity***
- This patient presents with **myasthenia gravis (MG)**, an autoimmune disorder affecting the neuromuscular junction through antibodies against acetylcholine receptors.
- **Extraocular muscles** and other muscles affected early in MG contain a high proportion of **fast-twitch (Type II) muscle fibers**, which are characterized by **high ATPase activity**.
- **Type II fibers** with high ATPase activity generate rapid, powerful contractions but are **more susceptible to neuromuscular junction dysfunction** due to their higher firing rates and greater dependence on efficient neuromuscular transmission.
- This intrinsic property (high ATPase activity) is why these muscles are preferentially affected in myasthenia gravis.
*A small mass per motor unit*
- While extraocular muscles do have **small motor units** (allowing for precise eye movements), this describes the **innervation pattern** rather than an intrinsic biochemical property of the muscle fibers themselves.
- The question specifically asks about an intrinsic property of the muscle group, referring to the metabolic and contractile characteristics of the muscle fibers.
*High myoglobin content*
- **High myoglobin content** is characteristic of **Type I (slow-twitch) oxidative fibers**, which rely on sustained oxygen delivery for prolonged, fatigue-resistant contractions.
- Muscles preferentially affected in MG have a higher proportion of **Type II fibers**, which have lower myoglobin content compared to Type I fibers.
*High density of mitochondria*
- **High mitochondrial density** is characteristic of **Type I (slow-twitch) oxidative fibers** that depend on aerobic metabolism for sustained energy production.
- While extraocular muscles do have oxidative capacity, the **Type II fibers** preferentially affected in MG have relatively lower mitochondrial density compared to Type I fibers and rely more on glycolytic metabolism for rapid energy needs.
*Increased amount of ATP generated per molecule of glucose*
- **Aerobic respiration** in Type I fibers generates approximately 32 ATP molecules per glucose through oxidative phosphorylation.
- **Type II fibers** rely more heavily on **anaerobic glycolysis**, which produces only 2 ATP per glucose molecule, making them less efficient in ATP generation per glucose.
- The muscles affected in MG have higher proportions of Type II fibers with lower ATP efficiency per glucose molecule.
Question 89: A 2-month-old girl with a previous diagnosis of DiGeorge syndrome is brought to the emergency department with her parents following a seizure. Her mother states that the baby had been inconsolable all day and refused to feed. She was born at 39 weeks gestation via spontaneous vaginal delivery. She is up to date on all vaccines. Upon arrival to the hospital her heart rate is 120/min, respiratory rate is 40/min, and temperature of 37.0°C (98.6°F). On examination, she is afebrile and somnolent and her fontanelles are open and soft. While attempting to take her blood pressure, the patient’s arm and hand flex sharply and do not relax until the cuff is released. A light tap on the cheek results in an atypical facial muscle twitch. A CMP and CBC are drawn and sent for analysis. Which of the following is the most likely cause?
A. Hypernatremia
B. Hypocalcemia (Correct Answer)
C. Tetanus
D. High fever
E. Meningitis
Explanation: ***Hypocalcemia***
- DiGeorge syndrome is associated with **thymic hypoplasia** and **parathyroid hypoplasia**, leading to **hypocalcemia** due to inadequate parathyroid hormone production.
- The symptoms described, including seizures, **Chvostek's sign** (facial muscle twitch with light tap on cheek), and **Trousseau's sign** (carpal spasm with blood pressure cuff inflation), are classic manifestations of **neuromuscular irritability** caused by severe hypocalcemia.
*Hypernatremia*
- **Hypernatremia** is characterized by excessive sodium levels, typically presenting with symptoms like extreme thirst, lethargy, confusion, or seizures due to **cerebral dehydration**.
- It does not cause the specific signs of neuromuscular excitability like Chvostek's or Trousseau's signs seen here.
*Tetanus*
- **Tetanus** is caused by the bacterium *Clostridium tetani* and manifests with painful muscle spasms, trismus (lockjaw), and opisthotonus, but does not present with Chvostek's or Trousseau's signs.
- Furthermore, the child is described as being up to date on all vaccinations, making tetanus unlikely.
*High fever*
- A **high fever** can cause febrile seizures in young children, but the patient is explicitly stated to be **afebrile** (37.0°C or 98.6°F), ruling out fever as the cause of the seizure.
- It also would not explain the other specific signs of neuromuscular irritability.
*Meningitis*
- **Meningitis** is an inflammation of the meninges, typically presenting with fever, irritability, lethargy, nuchal rigidity, and bulging fontanelles in infants.
- The patient is **afebrile** and has **soft, open fontanelles**, and her symptoms are more consistent with metabolic derangements rather than central nervous system infection.
Question 90: A 17-year-old teenager presents to the clinic with her parents complaining of headaches and loss of vision which began insidiously 3 months ago. She describes her headaches as throbbing, mostly on her forehead, and severe enough to affect her daily activities. She has not experienced menarche. Past medical history is noncontributory. She takes no medication. Both of her parents are alive and well. Today, her blood pressure is 110/70 mm Hg, the heart rate is 90/min, the respiratory rate is 17/min, and the temperature is 37.0°C (98.6°F). Breasts and pubic hair development are in Tanner stage I. Blood work is collected and an MRI is performed (the result is shown). Decreased production of which of the following hormones is the most likely explanation for the patient's signs and symptoms?
A. Antidiuretic hormone
B. Adrenocorticotropic hormone
C. Prolactin
D. Gonadotropins (Correct Answer)
E. Thyroid-stimulating hormone
Explanation: **Gonadotropins**
- The MRI image shows a **pituitary mass** (indicated by the red arrow) in a 17-year-old girl with **amenorrhea** (has not experienced menarche) and **Tanner stage I breast and pubic hair development**.
- This clinical picture, coupled with the mass effect, suggests **hypogonadotropic hypogonadism** due to compression of the pituitary gland, leading to insufficient production of **gonadotropins** (LH and FSH), which are essential for pubertal development and menstrual cycles.
*Antidiuretic hormone*
- Deficiency of ADH primarily causes **diabetes insipidus**, characterized by **polyuria** and **polydipsia**, which are not mentioned in this patient's symptoms.
- While a pituitary mass can affect ADH production, the primary clinical presentation here is related to pubertal delay.
*Adrenocorticotropic hormone*
- ACTH deficiency would lead to **adrenal insufficiency**, presenting with symptoms such as **fatigue, weakness, weight loss, hypotension, and electrolyte disturbances**. These symptoms are not described in the patient.
- Her vital signs (e.g., blood pressure) are within normal limits.
*Prolactin*
- Elevated prolactin (hyperprolactinemia) can cause **amenorrhea** and **galactorrhea** (if the patient has reached puberty), but it is usually due to a **prolactinoma** or stalk compression.
- While a pituitary mass could potentially affect prolactin, the lack of pubertal development (Tanner stage I) points more directly to a general gonadotropin deficiency rather than isolated prolactinopathy.
*Thyroid-stimulating hormone*
- TSH deficiency would result in **hypothyroidism**, characterized by symptoms like **fatigue, weight gain, cold intolerance, and bradycardia**.
- The patient's vital signs are normal, and symptoms of hypothyroidism are not described.
