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A 4-year-old boy presents to the emergency room with muscle spasms. His past medical history is significant for
recurrent infections and neonatal seizures. Evaluation of his serum electrolytes reveals hypocalcemia. This
patient would be most susceptible to which of the following diseases?
A. Chickenpox
B. Diphtheria
C. Gas gangrene
D. Gonorrhea
E. Tetanus
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A.Degorge syndrom,T cell dysfunction
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The correct answer is A. This boy has DiGeorge's syndrome, as evidenced by his tetany (muscle spasms) due
to hypocalcemia and his history of recurrent infections and neonatal seizures. The syndrome occurs because
of an embryonic failure in the development of the third and fourth pharyngeal pouches. Patients have both
hypoplastic parathyroids (producing hypocalcemia) and thymuses (producing T-cell deficiency and recurrent
infections). Since cell-mediated immunity (which depends on T cells) is important in defense against infections
caused by intracellular pathogens (such as viruses), patients with DiGeorge's are particularly susceptible to
viral infections, such as chickenpox (varicella). They also have trouble with fungal pathogens (e.g., Candida)
and mycobacteria.
Note that the USMLE might ask you other questions about DiGeorge's syndrome, so you should be able to
recognize other clues to the diagnosis, including: congenital cardiac defects, esophageal atresia, bifid uvula,
short philtrum, hypertelorism, antimongoloid palpebral slant, mandibular hypoplasia, and low-set ears.
Diphtheria (choice B) is caused by Corynebacterium diphtheriae, which produces disease by the elaboration
of a very potent exotoxin. Therefore, humoral immunity (antitoxin), which is not usually compromised in
DiGeorge's patients, is essential for defense against the organism. (Note that the C. diphtheriae exotoxin acts
by causing the ADP-ribosylation of elongation factor-2 of eukaryotic cells, thereby inhibiting protein synthesis).
The disease can be avoided by immunization with diphtheria toxoid.
Gas gangrene (choice C) is caused by Clostridium perfringens, which produces a potent alpha toxin that
injures cell membranes. Therefore, humoral immunity would again play a predominant role in defense against
this organism. Note that the disease occurs in wounds and would not be expected in an uninjured 4-year-old
boy.
Gonorrhea (choice D) is caused by Neisseria gonorrhoeae and would not be expected in a 4-year-old boy
unless there was evidence of sexual abuse. Virulence factors of this organism include pili, cell wall endotoxin
and outer membrane, and secretory IgA protease. Antibody responses, neutrophils, and complement are of
prime importance in defense against gonococcal infections.
Tetanus (choice E) is caused by Clostridium tetani and serves as a tricky distracter, as you might have quickly
associated the patient's muscle spasms with this answer choice. (This is why it is important to read the
question stem carefully before prematurely jumping to the responses). C. tetani, which gains entry through
deep wounds, produces tetanus toxin (exotoxin) and can be prevented by immunization with tetanus toxoid.
