**Core Concept**
Coagulase-negative staphylococci (CoNS) are a group of gram-positive bacteria that do not produce the enzyme coagulase, which is responsible for clotting blood. They are commonly found on the skin and mucous membranes and can cause infections, particularly in patients with compromised immune systems.

**Why the Correct Answer is Right**
Coagulase-negative staphylococci include species such as Staphylococcus epidermidis and Staphylococcus saprophyticus, which are known to cause infections in humans. These bacteria are often opportunistic pathogens that take advantage of breaches in the host's defenses to cause disease. They are commonly isolated from the skin, respiratory tract, and urinary tract.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Staphylococcus aureus is a coagulase-positive staphylococcus, not a coagulase-negative staphylococcus.
**Option B:** This option is incorrect because Staphylococcus lugdunensis is a coagulase-negative staphylococcus, but it is a less common species.
**Option C:** This option is incorrect because Staphylococcus haemolyticus is a coagulase-negative staphylococcus that is often isolated from human skin.
**Option D:** This option is incorrect because Staphylococcus saprophyticus is a coagulase-negative staphylococcus that is commonly isolated from the urine of healthy young women.

**Clinical Pearl / High-Yield Fact**
Coagulase-negative staphylococci are often resistant to multiple antibiotics, making them challenging to treat. Therefore, it is essential to identify the species of CoNS and choose the appropriate antibiotic therapy.

**Correct Answer: A. Staphylococcus aureus.**