Ease Into Prep with a USMLE^® Step 2 CK Question of the Day

The correct answer is D. This patient is demonstrating the classic picture of pneumococcal pneumonia. Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in this age group. The usual presentation is sudden onset of shaking chills, with rigors, high fever, and difficulty breathing. Pleuritic chest pain is often present and signifies bacterial infection. A white blood cell count, not provided in this case, most often is significantly elevated with a left shift (predominantly bands and polymorphonuclear cells). Chest radiographs usually reveal a lobar distribution of the pneumonia. Pleural effusions are present in up to 30% of the cases. Gram stain of the sputum commonly reveals gram-positive diplococci in chains.

Gram-negative diplococci (choice A) would be present in pneumonia due to Moraxella catarrhalis. This pathogen may produce acute pneumonia and usually occurs in the elderly or in those with a history of chronic bronchitis or obstructive lung disease. It is much less common than S. pneumoniae.

Gram-negative rods (choice B) are not a common cause of pneumonia in this population of patients. Gram-negative rods causing pneumonia include Klebsiella, Enterobacter, Serratia, and Proteus, which occur more commonly in patients who are debilitated, have a history of alcohol abuse, or are residing in nursing homes or similar institutions. These bacteria are often responsible for nosocomial pneumonias and, infrequently, community-acquired pneumonia.

Gram-positive cocci in clusters (choice C) that cause pneumonia are usually Staphylococcus aureus. S. aureus is an uncommon cause of community-acquired pneumonia. When it does cause disease, it is usually during or just following an epidemic of viral influenza. S. aureus may be seen year-round in the hospital, because it is a common cause of nosocomial pneumonia.

Gram-positive rods (choice E) would likely be Corynebacterium diphtheriae (diphtheria). This patient presents with pneumonia, not diphtheria (an infection that occurs in the pharynx, middle ear, larynx, skin, or bronchi).