An infant boy of unknown age and medical history is dropped off in the emergency department. The infant appears lethargic and has a large protruding tongue. Although the infant exhibits signs of neglect, he is in no apparent distress. The heart rate is 70/min, the respiratory rate is 30/min, and the temperature is 35.7°C (96.2°F). Which of the following is the most likely cause of the patient’s physical exam findings?

Congenital agenesis of an endocrine gland in the anterior neck

Autosomal dominant mutation in the SERPING1 gene

Genetic imprinting disorder affecting chromosome 11p15.5

Type I hypersensitivity reaction

Excess growth hormone secondary to pituitary gland tumor

A 16-year-old girl is brought to the emergency department unresponsive. A witness reports that she became anxious, lightheaded, and began sweating and trembling a few minutes before she lost consciousness. Her vitals are as follows: blood pressure 95/60 mm Hg, heart rate 110/min, respiratory rate 21/min, and temperature 35.5°C (95.5°F). She becomes responsive but is still somnolent. She complains of dizziness and weakness. A more detailed history reveals that she has drastically restricted her diet to lose weight for the past 18 hours, and has not eaten today. Her skin is pale, wet, and cold. The rest of the physical examination is unremarkable. Blood testing shows a plasma glucose level of 2.8 mmol/L (50.5 mg/dL). Which of the following statements is true?

Hypoglycemia in this patient is being compensated with an increased glycogenolysis rate.

Epinephrine-induced gluconeogenesis is the main process that allows for the compensation of a decreased glucose level.

There is an increase in the glycogen synthesis rate in this patient’s hepatocytes.

The patient’s symptoms are most likely the consequence of increased insulin secretion from the pancreatic islets.

The patient’s hypoglycemia inhibits glucagon release from pancreatic alpha cells.

You have been asked to deliver a lecture to medical students about the effects of various body hormones and neurotransmitters on the metabolism of glucose. Which of the following statements best describes the effects of sympathetic stimulation on glucose metabolism?

Epinephrine increases liver glycogenolysis.

Norepinephrine causes increased glucose absorption within the intestines.

Without epinephrine, insulin cannot act on the liver.

Peripheral tissues require epinephrine to take up glucose.

Sympathetic stimulation to alpha receptors of the pancreas increases insulin release.

A 42-year-old man undergoes therapeutic hypothermia (target temperature 33°C/91.4°F) following cardiac arrest with return of spontaneous circulation. During the cooling phase, he develops shivering, which increases oxygen consumption and interferes with target temperature achievement. He is already on sedation and neuromuscular blockade is being considered. Evaluate the most appropriate management strategy considering both efficacy and safety.

Increase sedation and add surface counter-warming of extremities before neuromuscular blockade

Administer meperidine alone to reduce shivering threshold

Abandon therapeutic hypothermia due to complications

Use only mechanical restraints to prevent movement

Immediate neuromuscular blockade without additional measures

Environmental stress responses for USMLE Step 2 CK: High-Yield Physiology Lessons

Heat Stress - When It's Too Hot to Handle

Pathophysiology: Occurs when the body's thermoregulatory mechanisms are overwhelmed by environmental heat load, leading to a dangerous rise in core body temperature.
Physiological Responses: Initial compensation involves peripheral vasodilation (shunting blood to the skin) and profuse sweating for evaporative cooling.
Spectrum of Illness:
- Heat Exhaustion: Core temperature is elevated but <40°C (<104°F). Profuse sweating, headache, nausea. Crucially, no CNS impairment.
- Heat Stroke: A medical emergency defined by core temperature >40°C (>104°F) AND central nervous system dysfunction (e.g., delirium, seizures, coma).

Heat exchange mechanisms and core/skin temperature changes

⭐ The defining feature separating heat stroke from heat exhaustion is altered mental status. Classic (non-exertional) heat stroke may present with anhidrosis (dry skin), while exertional heat stroke patients are often still sweating.

Cold Stress - The Big Chill

Primary Goal: Conserve heat & ↑ heat production.
Coordinating Center: Posterior hypothalamus.

Physiological Responses:

Acute (Immediate):
- Peripheral Vasoconstriction: ↑ Sympathetic tone (α1-receptors) diverts blood from skin to the core, reducing radiant heat loss.
- Shivering: Involuntary, rhythmic muscle contractions generate heat.
Chronic (Acclimatization):
- Non-Shivering Thermogenesis (NST): ↑ thyroid hormone & catecholamines stimulate brown adipose tissue (BAT).
- Mechanism: Uncoupling protein 1 (UCP1/Thermogenin) in BAT mitochondria produces heat instead of ATP.

⭐ Neonates rely heavily on non-shivering thermogenesis in brown fat for heat production, as they have a limited ability to shiver.

Thermoregulation: Hot vs. Cold Stress Response Pathways

Acclimatization - Body's Long Game

Heat Acclimatization (7-14 days): Gradual physiological adaptation to heat stress, improving exercise tolerance and reducing cardiovascular strain.
- Cardiovascular: ↑ Plasma volume, ↓ heart rate at a given workload, ↑ stroke volume.
- Sweating: Earlier onset, ↑ sweat rate, and significantly more dilute sweat (↓ NaCl loss).
Cold Acclimatization: Less pronounced in humans.
- ↑ Basal metabolic rate & non-shivering thermogenesis (brown fat).
- Enhanced peripheral vasoconstriction to conserve core heat.

⭐ High-Yield: During heat acclimatization, increased aldosterone enhances Na+ reabsorption by sweat gland ducts. This conserves electrolytes by making sweat hypotonic, a key adaptation for preventing exertional hyponatremia.

Fever vs. Hyperthermia - Thermostat Wars

Fever: Hypothalamic set-point is elevated. The body actively works to reach this new, higher temperature (e.g., via chills).
Hyperthermia: Set-point is normal. Heat production or absorption overwhelms heat loss mechanisms.

Feature	Fever (Pyrexia)	Hyperthermia
Set-Point	↑ Increased (via PGE₂)	Normal
Cause	Pyrogens (IL-1, IL-6)	Heat stroke, NMS, MH
Antipyretics	Effective	Ineffective
Treatment	Address cause, NSAIDs	Rapid external cooling

⭐ Antipyretics (NSAIDs) inhibit COX to block PGE₂ synthesis, resetting the hypothalamic thermostat. This mechanism is irrelevant in hyperthermia where the thermostat is already normal.

High‑Yield Points - ⚡ Biggest Takeaways

Heat stroke is a life-threatening emergency defined by CNS dysfunction (delirium, coma), unlike heat exhaustion.

Malignant hyperthermia, triggered by succinylcholine or halothane, requires immediate treatment with dantrolene.

Neuroleptic malignant syndrome (NMS) from antipsychotics presents similarly and also responds to dantrolene.

Frostbite involves ice crystal formation in tissues; manage with rapid rewarming and avoid rubbing.

Hypothermia (core temp <35°C) classically shows J waves (Osborn waves) on an EKG.

Acclimatization to heat stress involves earlier onset of sweating and less salt loss in sweat.

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