A 28-year-old woman is admitted to the hospital for a 2-week history of high, daily spikin… — USMLE Step 2 CK

A $28$ -year-old woman is admitted to the hospital for a $2$ -week history of high, daily spiking fevers up to $39.8^\circ\text{C}$ ( $103.6^\circ\text{F}$ ), severe pharyngitis, and polyarthralgia in her wrists and knees. During the fever spikes, she develops a transient, salmon-colored macular rash on her trunk and extremities. Initial laboratory studies on admission showed:

Laboratory Test	Result	Reference Range
Leukocytes	$18,500/\text{mm}^3$	$4,500-11,000/\text{mm}^3$
Hemoglobin	$11.2\text{ g/dL}$	$12.0-15.5\text{ g/dL}$
Platelets	$450,000/\text{mm}^3$	$150,000-400,000/\text{mm}^3$
ESR	$105\text{ mm/h}$	$0-20\text{ mm/h}$
CRP	$140\text{ mg/L}$	$<10\text{ mg/L}$
Ferritin	$5,500\text{ ng/mL}$	$15-150\text{ ng/mL}$

She is treated with high-dose nonsteroidal anti-inflammatory drugs. On day $4$ of hospitalization, she rapidly becomes lethargic and hypotensive. Physical examination now reveals marked hepatosplenomegaly. Repeat laboratory studies show:

Laboratory Test	Result	Reference Range
Leukocytes	$2,100/\text{mm}^3$	$4,500-11,000/\text{mm}^3$
Hemoglobin	$8.4\text{ g/dL}$	$12.0-15.5\text{ g/dL}$
Platelets	$42,000/\text{mm}^3$	$150,000-400,000/\text{mm}^3$
AST	$850\text{ U/L}$	$8-40\text{ U/L}$
ALT	$720\text{ U/L}$	$7-56\text{ U/L}$
Fasting triglycerides	$520\text{ mg/dL}$	$<150\text{ mg/dL}$
Fibrinogen	$85\text{ mg/dL}$	$200-400\text{ mg/dL}$
ESR	$12\text{ mm/h}$	$0-20\text{ mm/h}$
CRP	$165\text{ mg/L}$	$<10\text{ mg/L}$
Ferritin	$45,000\text{ ng/mL}$	$15-150\text{ ng/mL}$

Which of the following is most likely to be found on further evaluation of this patient's current condition?

Bone marrow demonstrating active engulfment of hematopoietic cells by macrophagesAnswer
B
Peripheral blood smear demonstrating abundant schistocytes and helmet cells
C
Positive blood cultures identifying a gram-negative rod organism
D
Markedly decreased serum ADAMTS13 protease activity
E
Bone marrow demonstrating severe hypocellularity with extensive fatty infiltration

Answer

Bone marrow demonstrating active engulfment of hematopoietic cells by macrophages.

This patient's acute deterioration is due to Macrophage Activation Syndrome (MAS), a severe and potentially fatal form of secondary hemophagocytic lymphohistiocytosis (HLH). Her initial presentation (quotidian fevers, salmon-colored rash, pharyngitis, polyarthritis, extreme hyperferritinemia) is classic for Adult-Onset Still's disease (AOSD). The acute transition characterized by the sudden onset of pancytopenia (from baseline leukocytosis), hepatosplenomegaly, transaminitis, and soaring ferritin levels is the hallmark of MAS. A critical, high-yield diagnostic clue for MAS is the paradoxical drop in the erythrocyte sedimentation rate (ESR) in the setting of a persistently elevated C-reactive protein (CRP). This occurs because widespread macrophage activation leads to hepatic dysfunction and fibrinogen consumption (hypofibrinogenemia); since ESR relies on fibrinogen to facilitate red blood cell rouleaux formation, the ESR plummets. To confirm MAS, a bone marrow biopsy is typically performed, which characteristically reveals hemophagocytosis (macrophages actively engulfing erythrocytes, leukocytes, and platelets).

Step-by-Step Solution

Analyze the initial clinical presentation.

Spiking fevers, salmon-colored rash, polyarthralgia, severe hyperferritinemia, and leukocytosis.

These classic findings strongly suggest Adult-Onset Still's disease (AOSD), establishing the baseline inflammatory condition.

Evaluate the clinical deterioration and new laboratory findings.

The patient developed pancytopenia, severe transaminitis, hypertriglyceridemia, hypofibrinogenemia, and a massive ferritin spike.

This multi-system organ failure pattern points toward an aggressive systemic inflammatory storm, specifically Macrophage Activation Syndrome (MAS).

Interpret the paradoxical drop in Erythrocyte Sedimentation Rate (ESR).

The ESR dropped from

105\text{ mm/h}

12\text{ mm/h}

, while the CRP remained elevated.

ESR depends heavily on circulating fibrinogen. In MAS, macrophages consume fibrinogen, leading to hypofibrinogenemia and an artificially low ESR despite worsening inflammation.

Identify the diagnostic hallmark of the complication.

The clinical picture is secondary hemophagocytic lymphohistiocytosis (HLH), diagnosed by demonstrating hemophagocytosis in the bone marrow.

MAS is a life-threatening complication characterized by uncontrolled activation of macrophages, requiring urgent identification and immunosuppression.

Key Concept

Macrophage Activation Syndrome (MAS)

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