USMLE Step 1 Question 355

Question:

A patient has fever, malaise, rhinorrhea, and conjunctivitis for several days and thinks he has come down with the common cold. A few days later, he develops small, punctate, bluish-white dots on a red base on his tongue and this is followed by a maculopapular rash starting on face and spreading to torso and limbs. Which of the following is the most likely diagnosis?

Correct Answer: A.  Infection with a highly contagious, single-stranded RNA paramyxovirus

This patient has measles, which is an acute, infectious, and highly contagious viral infection caused by morbillivirus, a paramyxovirus, contacting the nasopharnx and bronchi and spreading to regional lymph nodes and systemically.

A tick-borne disease caused by Rickettsia rickettsii is Rocky Mountain Spotted Fever. The characteristic rash in this disease starts 3-4 days after onset of symptoms as pink flat macules on wrists and ankles. The rash then spreads to proximal arms, legs, torso, and finally to palms and soles before becoming hemorrhagic, petechial, and nonblanching.

An infection with a togavirus includes rubella or german measles. The characteristic rash in this infection also starts on the face and moves down the to the torso, but you would not see Koplik’s spots in this infection. In addition, rubella is usually subclinical and when it does present, it is often a milder infection than measles or rubeola.

Smallpox is an infection with a double-stranded DNA variola virus. The rash seen in this infection starts on the face, in the mouth, and on arms before spreading to the rest of the body. It is a macular rash that progresses to vesicles and pustular lesions in 1-2 weeks with scabbing. The only risk of smallpox is from bioterrorism, so this would be a much less likely cause of disease.

Infection caused by the release of toxins by Staphylococcus aureus is toxic shock syndrome. This infection results in acute onset of fever, vomiting, diarrhea, headache, sore throat, strawberry tongue, and erythematous mucous membranes (conjunctiva, oropharynx, vagina). The rash commonly seen is a scarlatina skin rash starting 5-7 days after symptom onset followed by desquamation occurring 7-10 days after onset on palms and soles. This is infection often has a rapid progression to multisystem organ dysfunction.