Question

A man presents to a STD clinic with urethritis and urethral discharge. Gram stain shows numerous pus cells but no microorganism. The culture is negative on routine laboratory media. The most likely agent is

• A. Chlamydia trachomatis
• B. Haemophilus ducreyi
• C. Treponema pallidum
• D. Neisseria gonorrhoeae

Hint:

No organism on Gram stain plus a negative routine culture in a man with urethritis points to Chlamydia trachomatis, an obligate intracellular bacterium.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The man has urethritis with discharge, but two facts stand out: the Gram stain shows plenty of pus cells with no visible organism, and routine bacterial culture is negative. We need an organism that causes urethritis yet cannot be seen on Gram stain or grown on ordinary media.

Step 2: Key Concept:
Urethritis in men is broadly split into gonococcal urethritis, caused by Neisseria gonorrhoeae, and non-gonococcal urethritis, most often caused by Chlamydia trachomatis. Chlamydia is an obligate intracellular bacterium, so it cannot complete its life cycle outside a host cell and will not grow on routine bacteriological media, and being so small it is not seen on a standard Gram stain.

Step 3: Detailed Explanation:
Neisseria gonorrhoeae is a Gram negative diplococcus that sits inside neutrophils, so a Gram stain of gonococcal urethritis classically shows intracellular Gram negative diplococci, and it grows on selective chocolate based media such as Thayer Martin agar. Since this Gram stain shows no organism at all and the culture is negative, gonorrhoea is not the best fit, ruling out option (4).
Haemophilus ducreyi causes chancroid, a painful genital ulcer with tender lymphadenopathy, not primarily urethritis, and when cultured it needs a special enriched chocolate agar with vancomycin, and would appear as small Gram negative bacilli in chains if seen. This does not fit the picture, ruling out option (2).
Treponema pallidum causes syphilis with a painless chancre, and it cannot be Gram stained or grown on any artificial culture medium at all, it needs dark field microscopy for direct visualisation. Its presentation does not match urethral discharge with pus cells, ruling out option (3).
Chlamydia trachomatis fits every clue: it is a well known cause of non-gonococcal urethritis, it does not stain well on Gram stain since it is an obligate intracellular organism, and it will not grow on routine bacterial culture media, needing cell culture or nucleic acid tests instead.

Step 4: Final Answer:
Pus cells with no organism seen and a negative routine culture point to an intracellular organism that cannot be cultured on ordinary media.
Hence, the correct answer is option (1), Chlamydia trachomatis.