An iatrogenic problem is harm that arises from medical care or interventions. It includes complications caused by a treatment, such as a medication side effect, a surgical complication, or an infection that develops as a result of care. That’s why describing it as a problem that occurs as a result of medical treatment is the best match. Infections can be related to care but aren’t defined by themselves, and issues from lifestyle choices or environmental exposures fall outside the realm of medical treatment.

Acidosis is about an increased acidity in the body's fluids—more hydrogen ions in the blood and extracellular tissues, which lowers pH. This emphasizes the presence of acids rather than pointing to a single cause. While high CO2 can drive acidosis because it forms carbonic acid, acidosis isn’t defined as high CO2 alone; metabolic acidosis can occur from bicarbonate loss or excess acid production even with normal CO2. Likewise, bicarbonate deficiency is a contributor to acidosis, but acidosis itself is about overall acidity, not just a lack of bicarbonate. So the best description is a state of greater-than-usual concentration of acidic substances in the blood and cells.

The resting membrane potential is negative because the cell interior carries a net negative charge compared with the outside. This arises from impermeant anions inside the cell (like certain proteins and phosphates) and the membrane’s greater permeability to potassium, which tends to diffuse out of the cell. As K+ leaves, positive charge exits more readily than it enters, leaving behind more negative charges inside. The Na+/K+ pump helps maintain the ion gradients but does not make the sides equal; it supports the conditions that allow K+ efflux to dominate. The other statements don’t fit: more cations inside would make the interior positive, water permeability doesn’t establish the voltage, and pumps maintaining equal numbers would erase the gradient that generates the negative potential.

Chvostek's sign shows increased neuromuscular excitability from hypocalcemia. When the clinician taps the facial nerve near the cheek, the facial muscles briefly twitch, especially around the mouth and eye. This response happens because low calcium lowers the threshold for nerve depolarization, making muscles contract more easily in response to a light stimulus. The description in the option—touching the cheek triggering involuntary facial contraction due to hypopolarization—captures this phenomenon, which is why it’s the best choice. Other described signs don’t involve tapping the facial nerve and producing a facial muscle twitch, so they don’t fit the characteristic response of Chvostek's sign.