A 71-year-old woman is brought to the emergency department from a skilled nursing facility… — USMLE Step 2 CK

A 71-year-old woman is brought to the emergency department from a skilled nursing facility due to progressively worsening altered mental status over the past 24 hours. Her medical history includes chronic obstructive pulmonary disease, type 2 diabetes mellitus, and hypertension. Vital signs are temperature $38.9^\circ\text{C}$ ( $102.0^\circ\text{F}$ ), blood pressure 82/50 mmHg, pulse 118/min, and respirations 26/min. Oxygen saturation is 94% on room air. Physical examination reveals dry mucous membranes and suprapubic tenderness. The lungs are clear to auscultation bilaterally. Cardiac examination shows tachycardia with a regular rhythm and no murmurs.

Laboratory and diagnostic results are as follows:

Test	Result	Reference Range
Sodium	$138\text{ mEq/L}$	136-145 $\text{mEq/L}$
Potassium	$4.8\text{ mEq/L}$	3.5-5.0 $\text{mEq/L}$
Chloride	$102\text{ mEq/L}$	95-105 $\text{mEq/L}$
Bicarbonate	$12\text{ mEq/L}$	22-28 $\text{mEq/L}$
Blood urea nitrogen (BUN)	$45\text{ mg/dL}$	7-18 $\text{mg/dL}$
Creatinine	$2.1\text{ mg/dL}$	0.6-1.2 $\text{mg/dL}$
Serum lactate	$5.2\text{ mmol/L}$	0.5-1.0 $\text{mmol/L}$
Thyroid-stimulating hormone (TSH)	$0.2\text{ \mu U/mL}$	0.4-4.0 $\text{\mu U/mL}$
Free thyroxine (T4)	$0.7\text{ ng/dL}$	0.9-1.7 $\text{ng/dL}$

Arterial blood gas (room air):

Parameter	Result
pH	$7.28$
PaCO2	$26\text{ mmHg}$
PaO2	$88\text{ mmHg}$

Urinalysis shows 3+ leukocyte esterase, positive nitrites, and >50 leukocytes/hpf. A central venous catheter is placed, and aggressive intravenous fluid resuscitation is initiated. Blood and urine cultures are pending.

Which of the following is the most appropriate next step in management?

Administer intravenous ceftriaxoneAnswer
B
Administer intravenous levothyroxine
C
Administer intravenous sodium bicarbonate
D
Administer intravenous furosemide
E
Perform endotracheal intubation

Answer

The correct action is to administer intravenous ceftriaxone, as the patient is in septic shock and requires immediate empiric antibiotic therapy while fluid resuscitation continues.

The correct answer emphasizes treating the fundamental cause of the patient's critical condition. She is in septic shock secondary to a urinary tract infection, evidenced by fever, hypotension, lactic acidosis, and a markedly abnormal urinalysis. The immediate priorities in septic shock are restoring perfusion (intravenous fluids) and eliminating the source of infection (empiric broad-spectrum antibiotics, such as ceftriaxone, targeting gram-negative enteric organisms). All of her other derangements (acidosis, altered thyroid labs) are secondary to this primary process.

Step-by-Step Solution

Diagnose the primary clinical syndrome based on vital signs and localizing laboratory data.

The patient has fever, hypotension, tachycardia, and a urinalysis positive for leukocyte esterase and nitrites, confirming a diagnosis of septic shock secondary to urosepsis.

Identifying the primary cause of instability dictates the most urgent life-saving interventions: fluids and empiric antibiotics.

Analyze the acid-base status using the arterial blood gas and basic metabolic panel.

The anion gap is

138 - (102 + 12) = 24

, indicating a high anion gap metabolic acidosis (due to a lactate of

5.2\text{ mmol/L}

). Using Winter's formula (

1.5 \times 12 + 8 \pm 2

), the expected PaCO2 is

24-28\text{ mmHg}

. The measured PaCO2 is

26\text{ mmHg}

, meaning the respiratory compensation is perfect.

Calculating expected compensation is crucial to determine if a secondary respiratory disorder exists that might require mechanical ventilation or other airway interventions.

Interpret the abnormal thyroid function tests in the context of the current clinical picture.

The low TSH and low free T4 represent euthyroid sick syndrome (non-thyroidal illness syndrome), a physiologic down-regulation of the hypothalamic-pituitary-thyroid axis during severe acute illness.

Preventing inappropriate treatment for laboratory abnormalities that are secondary to the primary critical illness avoids iatrogenic harm.

Key Concept

In patients presenting with septic shock and multiorgan derangements, management must prioritize reversing the underlying infection and hypoperfusion (fluids and antibiotics) while recognizing appropriate physiologic adaptations like respiratory compensation for metabolic acidosis and euthyroid sick syndrome.

Estimated Time:1m 30s

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