A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 21-year-old college student is admitted to the emergency department with complaints of pharyngitis, headache, and a persistent, non-productive, dry, hacking cough. The patient complains of feeling tired and fatigued and denies fever/chills. On physical examination, her mucosa is pale. A complete blood count is remarkable for decreased hemoglobin. The physician suspects viral pneumonia, but the sputum culture tests come back with the following description: ‘fried-egg shaped colonies on sterol-containing media, and mulberry-shaped colonies on media containing sterols’. A direct Coombs test comes back positive. Which of the following statements is true regarding the complications associated with Mycoplasma pneumoniae?
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A
Red blood cells bind to IgG in warm temperatures > 37°C (98.6°F)Incorrect. Describes warm AIHA, in which IgG binds RBCs optimally at 37 degrees C; this is the mechanism in SLE and CLL, not Mycoplasma infection.
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B
It is similarly associated with systemic lupus erythematosusIncorrect. SLE is classically associated with warm (IgG-mediated) AIHA, not cold agglutinin disease, so this comparison is incorrect.
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C
Red blood cells bind to IgM in cold temperatures < 37°C (98.6°F)Correct. cold agglutinin disease from Mycoplasma involves IgM antibodies against the RBC I antigen that bind in cooler peripheral temperatures and fix complement.
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D
The underlying mechanism is complement-independent.Incorrect. cold agglutinin hemolysis IS complement-dependent; IgM is highly efficient at activating the classical complement pathway.
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E
Hemolysis is mediated by direct antibody-induced osmotic lysis without complement or splenic involvementIncorrect. Describes a mechanism of intravascular lysis without complement; Mycoplasma-associated cold agglutinin disease relies on complement activation by IgM and is not a direct osmotic process.
↑ Tap an answer to reveal the reasoning
Answer: C. The clinical picture (young patient, dry hacking cough, pharyngitis, low-grade illness, pale mucosa with anemia, and culture showing fried-egg/mulberry-shaped colonies on sterol-containing media) points to atypical pneumonia from Mycoplasma pneumoniae complicated by cold agglutinin-mediated autoimmune hemolytic anemia. The positive direct Coombs test plus anemia in this context is the classic Mycoplasma extrapulmonary complication.
M. pneumoniae infection induces IgM autoantibodies (cold agglutinins) directed against the I antigen on red blood cell membranes. These IgM antibodies bind RBCs at temperatures below core body temperature (cooler peripheral circulation), fix complement, and cause intravascular and extravascular hemolysis. This is a complement-dependent process — exactly opposite of option D.
Distinguish this from warm autoimmune hemolytic anemia (IgG-mediated, binds at 37 degrees C, classically associated with SLE and CLL), which would NOT explain the post-Mycoplasma picture. Treatment focuses on the underlying infection with a macrolide, doxycycline, or fluoroquinolone; severe hemolysis is managed with warming and avoidance of cold exposure.
**Why each option:**
**A.** Describes warm AIHA, in which IgG binds RBCs optimally at 37 degrees C; this is the mechanism in SLE and CLL, not Mycoplasma infection.
**B.** SLE is classically associated with warm (IgG-mediated) AIHA, not cold agglutinin disease, so this comparison is incorrect.
**C.** Correct — cold agglutinin disease from Mycoplasma involves IgM antibodies against the RBC I antigen that bind in cooler peripheral temperatures and fix complement.
**D.** Incorrect — cold agglutinin hemolysis IS complement-dependent; IgM is highly efficient at activating the classical complement pathway.