**Core Concept**
Nosocomial infections, also known as hospital-acquired infections (HAIs), are infections that patients acquire during their stay in a hospital or healthcare facility. These infections can be caused by a variety of factors, including contaminated medical equipment, healthcare worker contamination, or the patient's own flora.

**Why the Correct Answer is Right**
Nosocomial infections are typically acquired after a patient has been admitted to the hospital. This is because the hospital environment provides an opportunity for the transmission of pathogens, which may not have been present in the patient before admission. The development of nosocomial infections usually occurs within 48 hours of admission, as the patient is exposed to potential sources of infection. Therefore, option D, "May already present at the time of admission," is incorrect because it contradicts the definition of nosocomial infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is true, as nosocomial infections can manifest within 48 hours of admission. This is a common timeframe for the development of HAIs.
**Option B:** This statement is also true, as nosocomial infections can develop after a patient has been discharged from the hospital. This can occur if the patient is exposed to the same pathogen in the community, or if the infection was not fully resolved before discharge.
**Option C:** This statement is true, as nosocomial infections denote a new condition that is unrelated to the patient's primary conditions. HAIs are typically caused by healthcare-related factors, rather than the patient's underlying medical condition.

**Clinical Pearl / High-Yield Fact**
It's essential for healthcare workers to recognize the signs and symptoms of nosocomial infections, as these infections can have significant morbidity and mortality. By implementing infection control measures and adhering to strict sterile technique, healthcare workers can reduce the risk of HAIs and improve patient outcomes.

**✓ Correct Answer: D. May already present at the time of admission**