A 68-year-old man comes to the clinic due to a 3-month history of progressively worsening … — USMLE Step 2 CK

A $68$ -year-old man comes to the clinic due to a $3$ -month history of progressively worsening lower back pain and severe fatigue. He also reports new-onset constipation over the past two weeks. He has no prior medical history and takes no medications. Vital signs are within normal limits. Physical examination shows pallor and tenderness to palpation over the lumbar spine. Laboratory studies show:

Parameter	Result
Hemoglobin	$9.2$ g/dL
Leukocyte count	$6,500$ /mm³
Platelet count	$210,000$ /mm³
Calcium	$11.9$ mg/dL
Creatinine	$2.3$ mg/dL
Total protein	$10.2$ g/dL
Albumin	$3.8$ g/dL

A peripheral blood smear demonstrates red blood cells that appear stacked on top of one another. Which of the following is the most likely diagnosis?

Multiple myelomaAnswer
B
Primary hyperparathyroidism
C
Osteomyelitis of the lumbar spine
D
Metastatic prostate cancer
E
Colorectal carcinoma

Answer

The most likely diagnosis is multiple myeloma.

This patient presents with a classic textbook case of multiple myeloma. The diagnosis is strongly supported by the presence of the 'CRAB' criteria: hyperCalcemia (

11.9

mg/dL), Renal insufficiency (creatinine

2.3

mg/dL), Anemia (hemoglobin

9.2

g/dL), and Bone pain (lumbar tenderness). Additionally, the laboratory results show a significantly elevated total protein with a normal albumin level, creating a large 'protein gap' (

10.2 - 3.8 = 6.4

g/dL). A protein gap greater than

4

g/dL is highly suggestive of the presence of a monoclonal immunoglobulin (M-protein) spike, which is the hallmark of plasma cell dyscrasias. Furthermore, the peripheral blood smear describes red blood cells stacked like coins, which is known as rouleaux formation. Rouleaux occurs because the high levels of circulating monoclonal proteins neutralize the negative charge on the surface of red blood cells, causing them to adhere to one another. The combination of these clinical and laboratory findings makes multiple myeloma the definitive diagnosis.

Step-by-Step Solution

Analyze the patient's clinical symptoms and physical examination findings.

The patient has progressive lower back pain, severe fatigue (pallor), and constipation, accompanied by localized lumbar spine tenderness.

These findings suggest potential bone involvement (lytic lesions), anemia, and hypercalcemia (which causes constipation and fatigue).

Interpret the provided laboratory values and the peripheral blood smear.

Labs reveal anemia (

9.2

g/dL), hypercalcemia (

11.9

mg/dL), and renal insufficiency (creatinine

2.3

mg/dL). There is a very large protein gap of

6.4

g/dL (Total protein

10.2

- Albumin

3.8

). The smear shows rouleaux formation.

This specific combination of lab abnormalities represents the 'CRAB' criteria and indicates severe hyperproteinemia, highly suspicious for a monoclonal gammopathy.

Synthesize the findings to determine the most likely underlying disease process.

The constellation of CRAB symptoms, a large protein gap, and rouleaux formation points definitively to multiple myeloma.

Multiple myeloma is a plasma cell dyscrasia characterized by the overproduction of monoclonal immunoglobulins, leading to bone destruction, hypercalcemia, renal impairment, and marrow replacement.

Key Concept

Multiple myeloma classically presents with the 'CRAB' criteria: hyperCalcemia, Renal insufficiency, Anemia, and Bone pain (due to lytic lesions). A large protein gap (total protein minus albumin >

4

g/dL) and rouleaux formation on a blood smear strongly support the diagnosis.

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